“Roach coach” is an unfortunate nickname for many taco trucks. In the 1970s, airlines serving Latin America were sometimes called “roach coaches” (a slur against the passengers). The term “roach coach” was applied to food trucks by at least the 1980s.
 
   
Wiktionary
roach coach
(idiomatic) Whimsically, a catering or food truck.
   
Wikipedia: Food truck
A food truck or mobile kitchen is a mobile venue that sells food. Some, including ice cream trucks, sell mostly frozen or prepackaged food; others are more like restaurants-on-wheels.

Food trucks make frequent appearances at carnivals, construction sites, and other temporary venues where large numbers of people gather. They can also be found at or near college campuses, office complexes, auto repair shops, movie sets and military bases, and often boast loyal followings.
 
In the United Kingdom, these are known as snack vans and can be found on nearly all major trunk roads at the side of the road selling their food. A 1/4lb burger can be purchased for about £2 (approx. $3.5USD). Many people prefer to stop at one of these Burger vans when travelling due to the cheap price, rather than stop at a motorway service station where prices can be extremely high. In anglophone Canada, they are known as ‘chip wagons’. In the US, they may whimsically be called ‘roach coaches’.
 
An early version of the food truck was military field kitchen - for example the US Army’s mobile canteen; another predecessor in the United States was the old West’s chuckwagon.

Ethnic food trucks
Taco trucks are mobile kitchens that primarily serve Mexican food. They are characteristic of cities in the United States with large Latino populations, mainly the southwest and especially southern California. They can be found, most often, in the inner city, parked in private parking lots or on public streets in commercial areas. Taco trucks have been mentioned in the Zagat Survey and the Los Angeles Times.
 
Urban Dictionary
1.  roach coach
A catering truck, especially one which frequents blue collar places of work; generally a modded pickup truck with insulated diamond-pattern doors covering a refrigerator case and a warmer unit with attached griddle.
“Naw, we wait till the roach coach comes around at 12:15 to take our lunch breaks around here.”
by Brian X Apr 18, 2004
2.  roach coach
the newer, industrial strength roach coach is a mini-restaurant on wheels that is mounted on a truck chassis and is about the size of a UPS truck. Typical coaches have sides that open up to reveal varions snack and beverage items, in addition to an order window where the smiling Hispanic gentleman and his wife/sister or other female cook take and fill orders. Previously confined to construction sites and garages, now the roach coach prowls office parks offering secretaries a fried breakfast and lunch. The call of the wild roach coach is an obnoxious semi-musical horn that calls the willing sheep to be sheared and then fattened up on grease-laden fare.
I ‘ate’ at the roach coach yesterday and had the trots all night.
When people bring in roach coach food, you can smell the grease all over the building.
Roach coach cooking is the opposite of good food.
by adam_before_eve Dec 28, 2005  
3.  roach coach
Roach Coaches are food service trucks, usually found making stops in industrial areas and county fairs. Some of the food can be quite good, but whatever you do, don’t eat the sushi!
I had some roach coach sushi, and I havent gotten off the toilet since.
by gooberliberation Feb 7, 2006
 
(CATNYP - New York Public Library record)
Author: United States. Naval Support Activity (Ða N˜ang, Vietnam)
Title: Naval support I-Corps Vietnam; Danang, Cua Viet-Dong Ha, Tan My-Hue, Phu Bai, Chu Lai, Sa Huynh.
Imprint: [Shinjuku-ku, Tokyo, Printed by Dai Nippon Print. Co., 1970?]
Pg. 88:
Its Food Services Branch operates both gallies and mobile food vans known jokingly as “roach coaches.” These mobile gallies serve 3000 hot meals daily throughout the Danang area.
 
24 January 1971, New York (NY) Times, “All Aboard for the ‘Cattle Car’” by Victor perera, pg. XX1:
PUERTO RICANS themselves call it the “cattle car” because of its reputation for overcorwding from the old days. Snobbish stewardesses refer to it as the “roach coach” because they expect the passengers to be unclean. A San Juan travel agent indelicately calls it the “vomit comet”—an allusion, I imagine, to the great number of babes in arms of inexperienced travelers aboard the plane.
 
6 October 1985, Washington (DC) Post, “The Shuttle: Everybody Is Nobody on the Eastern Power Trip” by David Remnick, pg. H10:         
Shuttle Nicknames
Roach Coach.
Cattle Car (courtesy Jacob Javits).
Glamor Glider.
Bagel-less Bomber.
Late Show.
Waiting Room.
Flying None.
 
Google News Archive
27 August 1984, St. Petersburg (FL) Times, pg. 3, col. 2:
He said carts used for preparing this type of product are difficult to maintain.
 
“We call them roach coaches,” he said.
 
10 October 1985, Washington (DC) Post, “Fay and Her Roach Coach, Breezing Along” by Barbara Carton, pg. D1:
For the last three years, she has driventhe traffic-clogged highways of Tysons Corner, dispensing snacks and lunches from “my roach coach.”
   
A roach coach is actually a food service truck, but neither the drivers nor the customers address it by its formal name.  And when Shannon speaks of hers, it’s happily, with affection—as in, “You ought to taste what a fine cup of coffee my roach coach can make,” or, “Today, this old roach coach is pretty much breezing along.”
 
23 March 1987, Los Angeles (CA) Times, “‘Hot Trucks’: They Keep It Clean and Feed a Need” by Jerry Belcher, Metro, pg. 1:
One labels his truck “Ye Olde Roach Coach.”
 
OCLC WorldCat record
Biker babes
by Trucker Wally  
Material Type: Audio book, etc.
Document Type: Sound Recording
Notes: Laughing Hyena Tapes: TC2025 (on insert). Additional title on insert: The best of Trucker Wally.
Description: 1 sound cassette : analog.
Contents: Liar size rubbers—Biker babes—Already broken freight—Driver weight loss systems—Bad ass truckers—Bob’s roach coach—Radar detector detector—Tanker load locks—Frequent fueler points—Sidewinder sporting missles—Diesel mechanics school—Dewey, Cheatum and Howe.
Series Title: Truckstop comedy, v. 25
 
24 January 1991, Houston (TX) Chronicle, “For new slang in desert, it’s still a military snafu” by Peter Copeland, pg. 15:
EASTERN SAUDI ARABIA - “Desert cherries” in “Kevlars” fly the “Sand Box Express” to the “beach” and soon are complaining about “Meals Rejected by Ethiopians” if they can’t find a “roach coach” run by “Bedouin Bob.”
 
In English, that would be new troops wearing helmets made of bullet-proof Kevlar taking military transport to the desert and complaining of the Meals, Ready-to-Eat (MRE) field rations if they can’t find a Saudi food stand.
     
8 July 1992, Los Angeles (CA) Times, “The Food Is Fine But It Isn’t the Place You’d Pick for a Picnic” by Tony Perry, pg. 1:
The dining establishment is called Tacos Mexico, which provides breakfast and lunch to hundreds of garbage truck drivers (municipal and private) and an occasional trash-hauling homeowner.
 
It may be San Diego’s most exclusive open-air dining experience. Certainly it is the most aromatic.
 
It begins with a catering truck, like hundreds that ply San Diego work sites. Yes, but this is no ordinary roach coach, ptomaine wagon or gedunk truck.
 
“With most of these trucks, you just get something to fill your stomach up,” said Henry Brown, a driver for the city’s Refuse Collection Division. “With this truck, they take time in preparing the food. We appreciate that.”
 
15 July 1992, Chicago (IL) Tribune, “Yuppie Catering Truck: A Yuppie Version of the Catering Truck is Rolling into California Office Parks” by Edwin Garcia:
A yuppie version of the catering truck is rolling into California office parks, offering a full veggie menu of pastas, granola and carrot juice that are low in fat, low in sugar, low in salt. “There are plenty of roach coaches in the area, so it’s not so foreign for people to come up to the truck to get something to eat. What’s new to them is getting something healthy,” said Laurene Powell, 28.
 
Powell and her business partner, John “J.J.” Mullane, 29, are Stanford University MBA grads who founded Terravera as an “appetizing alternative” to the typical “roach coach” company. Their Earth Cruiser, a large white van outfitted with a candy-striped canvas awning, has been serving about 100 meals daily since it was mobilized last month.
   
OCLC WorldCat record
Cruise of the roach coach.
by Paul Weiby
Type:  Book; English
Publisher: [S.l.] : Xlibris Corp, 1997
 
14 August 1997, Charlotte (NC) Observer, “Language of Lunch: Caterer’s Wagon Tells Hispanic Workers: Let’s Eat,” pg. 8E:
They call them roach coaches. But everything on the Rey Taco truck is sparkling clean.
 
30 November 1997, New York (NY) Times, “On film sets, no one works until the ‘roach coach’ arrives. So how good is the food?” by Eric Asimov, pg. ST1:
A day’s work cannot begin until the catering van, called the “roach coach,” rolls up.