One drawing depicted the area behind the wagon seat as being heaped with bagged goods. Coffee, bread, beans, and dried or salted meat were the main foods carried. Some food might be gathered along the route, but it was unlikely that the cowboys had fresh beef unless a cow became injured.
The cook was also the closest thing to a doctor on the trip. Considering the area at the time, cowboys from rural areas probably didn't have much different medical treatment even when not on a cattle drive.
The chuck wagon didn't just hold the medical supplies, it also held the cook's bedroll.
Imagine living for a month in this mobile home, through rain, wind, heat and cold—with a lot of cows being kept on the move—and having a crew riding along with little more privacy than a wide brimmed hat could provide.
Photos taken at Pawnee Bill's Museum, Oklahoma