The chuck wagon was declared by the Texas legislature to the the official state vehicle of Texas. Attached to the back of the chuck wagon was usually the “chuck box,” a cook’s pantry containing everything a cook needed to prepare a meal.
 
The “chuck wagon” was earlier called a “mess wagon,” and the similar term “mess box” appears in print earlier than “chuck box.”
             
 
(Dictionary of American Regional English)
chuck box n, also attrib chiefly West
A box for carrying provisions, usu attached to a chuck wagon.
1904 DN 2.396 [Lumbering terms], Chuck-box...A complete pantry attached to the rear of a cowboy’s camp wagon.
1933 AmSp 8.1.27 nTX, Chuck box. A sort of traveling kitchen cabinet affized to the back of the chuck wagon, having many pigeon-holes and compartments for stowing away food and utensils needed..at any time the outfit ha t ostay away from headquarters at meal time.
1944 Adams Western Words, Chuck-box—Bolted to the rear of the chuck wagon is the chuck-box. it has a hinged lid that, when let down and supported by a stout leg, forms a wide shelf or table. This is the cook’s private property, and woe unto the nervy puncher who tries to use it for a dining table. Occasionally this privilege is granted to the wrangler, who generally eats after all the others have finished and are changing horses, but never to a rider.
1956 Ker Vocab W. TX 193.
1966 DARE Tape NM13, Hinged and let down; that was the chuck box lid, and the cook was the private owner of that chuck box lid; that was his; NM14, Chuck box lid—almost a symbol for the cook’s privacy.
 
29 October 1896, Butte (MT) Weekly Miner, “The Cowboy As He Is,” pg. 6:
“Chuck wagon” means a provision wagon, and “chuck box” has a similar meaning.
 
28 September 1901, Atlanta (GA) Constitution, “Atlantan Who Drew Lot Returns From Oklahoma,” pg. 9, cols. 2-3:
“They are attracted by smelling the ‘chuck box.’ That’s the vernacular for provisions—‘chuck,’ they call it out there, meaning anything in the way of ‘rations’ or edibles.”
 
20 March 1908, San Antonio (TX) Daily Express,  pg. 6, col. 6:
I want to say to my old-time friends, who like myself, never attended a banquet other than one out of a “chuck box” in the “hind” end of a Studebaker wagon, and for the information of my friend, Tom Gooch, of The Express, I will say that a “chuck” wagon in nowise resembles a water wagon,... 
 
13 August 1910, Galveston (TX) Daily News, pg. 4, col. 3:
It was a real chuck wagon, though the regular chuck box as missing, and it is to be feared that the “chuck” which could have been furnished from the wagon would have aroused the righteous indignation of a crowd of real cowboys to such a degree that the cook would have been in imminent danger of finding himself dangling from the end of the wagon tongue which had been propped up with the breastyoke.
     
26 February 1928, Dallas (TX) Morning News, feature section, pg. 5:
When Cow-Camp Cooks Called “Chuck”
 
Maybe All the Boys Got to Eat Was Sour-Dough Bread and Beans But Gosh, How Good It Tasted!
 
The old-time autocratic camp “cookie” is fast disappearing, but in his day he was as necessary to the range as cows and absolutely ruled the roost through the chuck box, dishing out chow that wasn’t fancy, but “lined a feller’s ribs well.”
By Ramon F. Adams.
 
24 September 1934, San Antonio (TX) Express, “Chuck Wagon Supper Monday Launches Drive for Maintenance Fund,” pg. 16, col. 5:
One of the features of the supper is a chuck wagon loaded by the Witte Memorial Museum along with chuck box made by the late George W. Saunders, old trail driver. The chief cook, or “cocinero,” is Charles Wendland.
 
Google Books
Cowboy Lingo
by Ramon F. Adams
Boston, MA: Houghton Mifflin Books
1936
Pg. 147:
The box, made to fit on the rear of the chuck-wagon, and serving the purpose of a kitchen cabinet, was called the “chuck-box.” The canvas which some cooks used to stretch over the “chuck-box” to make shade and shelter was called a “fly.”
 
9 August 1936, Abilene (TX) Morning Reporter-News, “Texas Toady Says” by Mil, pg. 9, col. 1:
HOWDY, TEXANS, DO YOU KNOW THAT THE CHUCK BOX USED TODAY VARIES BUT LITTLE FROM THE ORIGINAL DESIGN? THE CHUCK WAGON, THE COWBOYS’ HOME ON WHEELS, IS A TEXAS INSTITUTION AND WAS USED AS A TRAVELING HOME BY COW OUTFITS ON THE TRAIL AND RANGE. CHARLES GOODNIGHT, A TEXAS PIONEER, IS CREDITED WITH DESIGNING AND BUILDING THE FIRST CHUCK BOX.
 
31 October 1943, San Antonio (TX) Light, “Monarchs of Chuck Wagon: Cooks Had to Be Tough Hombres” by J. Frank Dobie,” pg. 4B, col. 2:
Nobody was to enter the space between the fire and his chuck box on the end of the wagon; no man was to shake a saddle blanket near enough for the hair and dust to float into the kitchen space; no man was to ride his horse up close enough to throw dust into the pots.
   
COOK SUPREME
As the saying went, “The cook is boss supreme for sixty feet around the chuck box.”