A turnip truck/wagon conveys the image of a country bumpkin driving in to the big city. “I didn’t just fall off the turnip truck”—it’s usually expressed in the negative—means that one is not naive or uniformed.
 
“But John didn’t exactly come to town on a turnip truck” was written by George Smith and printed in the Anniston (AL) Star on January 18, 1970. “Either Roger Bear thinks I came in here on a turnip truck or he just plain doesn’t like sports writers” was written by George Smith and printed in the Anniston (AL) Star on May 7, 1970. “I may have been born on a turnip truck, but that doesn’t mean I’m still riding it” was written by George Smith and printed in the Anniston (AL) Star on October 9, 1970.
 
The expression of “falling/getting off” the turnip truck became popular in 1975. “What did you do, just fall off a turnip truck?“was written by Merrill Lockhard and printed in the Nashua (NH) Telegraph on July 16, 1975. “The latter, especially, apparently haven’t gotten off the turnip truck” was printed in the Independent (Long Beach, CA) on August 5, 1975. “Anybody who thinks a manager has ever won the World Series must have fallen off the turnip truck” was printed in the Austin (TX) American-Statesman on August 8, 1975.
 
“Turnip wagon” has been less frequently cited. “Don’t fall out of the turnip wagon” was printed in The Daily News (Huntingdon and Mount Union, PA) on July 29, 1985. “Mr. Goldin (New York City Comptroller Harrison J. Goldin—ed.) did not just fall off the turnip wagon” was printed in the Philadelphia (PA) Inquirer on June 20, 1976.
 
“Johnny Carson helped to make the ‘turnip truck’ expression famous” was posted on Twitter by Marinelle K. Szenasy on September 6, 2011. It’s not known when American comedian and television talk show host Johnny Carson (1925-2005) first said this on The Tonight Show.
 
   
Wiktionary: fall off the turnip truck
Verb
fall off the turnip truck
1. (chiefly US, idiomatic) To be naive, uninformed, or unsophisticated, in the manner of a rustic person.
         
18 January 1970, Anniston (AL) Star, “John And Baskets: Building A Winner At Wellborn High” by George Smith, pg. 1C, col. 1:
But John didn’t exactly come to town on a turnip truck and he’ll tell you now that watching fellows like Lou Scales and Walter Holt, Emma Sansom’s coach, did some changing in his thinking.
 
7 May 1970, Anniston (AL) Star, “This ‘N That” by George Smith, pg. 8B, col. 1:
Either Roger Bear thinks I came in here on a turnip truck or he just plain doesn’t like sports writers.
 
9 October 1970, Anniston (AL) Star, “This ‘N That” by George Smith, pg. 1B, col. 1:
I may have been born on a turnip truck, but that doesn’t mean I’m still riding it.
 
16 July 1975, Nashua (NH) Telegraph, “Whatshisname Advises Ford” by Merrill Lockhard, pg. 4, col. 5:
“What did you do, just fall off a turnip truck?”
(Spoken by the fictional Youall Goodweed.—ed.)
     
29 July 1975, The Daily News (Huntingdon and Mount Union, PA), “Dairymen Convene,” pg. 14, col. 6:
ALTOONA—“Don’t fall out of the turnip wagon,” Dr. James E. Honan, general manager of the Inter-State Milk Producers’ Cooperative told more than 700 members and guests during the summer meeting held today in Lakemont Park, Altoona.
 
5 August 1975, Independent (Long Beach, CA),“Lahoud keeps ‘em smiling” by Hank Hollingworth, pg. C1, col. 1:
The latter, especially, apparently haven’t gotten off the turnip truck.
 
8 August 1975, Austin (TX) American-Statesman, Kirk Bohls column, pg. 53, col. 2:
Anybody who thinks a manager has ever won the World Series must have fallen off the turnip truck.
 
2 October 1975, Cloverdale (CA) Reveille, “Chamber Chatter” by Vivian Weer, pg. 12, col. 4:
(Our Directors didn’t just fall off the turnip truck, you know…)
     
8 November 1975, The Republic (Columbus, IN), “Oh, the Bad Old Days Look So Good Today” by Dick West (UPI), pg. A3, col. 7:
“Those Arab oil producers didn’t just fall off the turnip truck, you know.”
 
27 November 1975, Atlanta (GA) Journal-Constitution, “Pulse of the Public: Troopers and Speeders,” pg. 35-A, col. 2:
As the old saying goes, “We didn’t just fall off the turnip truck.”
(...)
COL. HERMAN COFER
Commissioner
Georgia Dept. of Public Safety
     
20 June 1976, Philadelphia (PA) Inquirer, “The Scene: In the nation and the world” by Jack Severson, pg. 3-A, col. 1:
Mr. Goldin (New York City Comptroller Harrison J. Goldin—ed.) did not just fall off the turnip wagon.
 
Google Books
Basic Computer Games, Vol. II
By David H. Ahl
Morristown, NJ: Creative Computing Press; Fort Worth, TX: Published for Radio Shack
1980
Pg. 56:
“HEY, I DION’T JUST FALL OFF A TURNIP TRUCK, YA KNOW”
     
Google Books
Texas Crude:
The how-to on talkin’ Texan
By Ken Weaver
New York, NY: E.P. Dutton
1984
Pg. 10:
“I didn’t just fall off the turnip wagon.”
1. I have more intelligence than you perceive.
2. I’m no country bumpkin: my bucolic origins belie my urbane sophistication.
   
Google Books
Magnolia Nights
By Martha Hix
New York, NY: Zebra Books/Kensington Publishing Corporation
1988
Pg. 173:
“I didn’t just fall off the turnip wagon.”
 
Google Books
Weathering the Storm
By Elaine Camp
New York, NY: Silhouette Books
1988
Pg. 108:
“I didn’t just fail off the turnip truck.”
 
“There’s that rural talk I love so much.”
       
Google Groups: alt.fan.letterman
“fall off a turnip truck”
Mark Israel
10/3/96
Is it true that Johnny Carson often used the phrase “fall off a turnip truck”?  If so, what year did he use it first?  Lexicographers would be interested in any documentable use of this phrase before 1985.
 
Urban Dictionary
turnip truck
Metaphorical vehicle bringing rubes to Gotham. One who falls off the truck is hopelessly naive.
You know, I didn’t just fall off the turnip truck! I know stuff about the Big City.
by octopod May 29, 2004
 
OCLC WorldCat record
In the turnip truck
Author: David Shiflet
Publisher: Austin, Tex. : Eakin Press, ©2005.
Edition/Format:   Print book : English : 1st ed
Summary:
This book is about the small town in all of us. The author’s hilarious and insightful view of those innocent years of ages five to twelve takes a provocative look at the people and events that shape our lives.
       
Straight Dope Message Board
where does the phrase . . .
Sternvogel
04-15-2005, 05:44 PM
I’ve heard the phrase as “I didn’t just fall off the turnip truck”. Johnny Carson (http://www.yaelf.com/aueFAQ/miffllffturnptruck.shtml) apparantly had a hand in popularizing it, but the first person I remember hearing it from was Woody Harrelson as Woody Boyd on Cheers.
 
Urban Dictionary
just fell off the turnip truck
Naive. Implies rural unsohistication versus urban savviness.
“A Rolex for $50? Do you think I just fell off the turnip truck?”
#ignorance#innocence#unsophistication#naivete#rurality
by The Demon Prince November 14, 2006
 
OCLC WorldCat record
I fell off the turnip truck : God, the devil and new pair of shoes
Author: Olga Vesta Button
Publisher: United States : O.V. Button, ©2007.
Edition/Format:   Print book : Biography : English
 
OCLC WorldCat record
Turnip truck stops here.
Author: Arlyn Vierkant
Publisher: [Place of publication not identified] : Publishamerica Inc., 2007.
Edition/Format:   Print book : English
 
Twitter
Marinelle K. Szenasy
@lumberjack_75
Replying to @GreekOlive
@GreekOlive @indrapalparmar Johnny Carson helped to make the “turnip truck” expression famous. 😊
9:55 PM - 6 Sep 2011 from Granbury East, TX