Chicago to San Antonio in coach... should be interesting!
Lately, the Texas Eagle has a pretty good on-time performance record, often arriving into San Antonio early... I hope that is the case for you.
I know that you can bring your own food and sodas with you in a small cooler, but health laws prevent Amtrak from heating up your food in their microwaves, etc... so don't bring anything you have to warm up to be palatable. Otherwise, the lounge car has a wide variety of snacks (candy, chips, cookies), meals (breakfast items, hot dogs, pizza, lasagna), and beverages (milk, sodas, beer, wine) available from morning until late evening.
Also, I wouldn't count on being able to get ice for your cooler throughout the trip. You won't be able to get it on-board in quantity and I can't recall any of the longer station stops having ice available along the route.
Also important, there is no dining car service between Fort Worth and San Antonio... the only food service available to coach passengers is in the lounge car after Fort Worth. So if you are planning to eat a meal in the diner, be sure you do so before Fort Worth!
For scenery purposes, it is better to sit toward the rear of the car, but since the coach seating is reserved, you may not have a choice of where you are seated in your coach. If there coach isn't too crowded, you may be able to move around a bit.
However, you can go to the sightseeing lounge car when you want to see the sights, as seating there is unreserved and open to all on board.
The Texas Eagle operates with two different consists, depending on whether the rear portion of the train is through-service to LA (3 days a week).
On most days, it is 1 locomotive, baggage car, transition sleeper (partly for the crew, partly for paying customers), dining car, sightseeing lounge, and two coaches, and sometimes a sleeper either on the end or in between the transition sleeper and dining car. I say this because I recently saw the train with a sleeper on the rear end on a non-LA through-service day. On my most recent trip two weeks ago, I don't recall there being a revenue sleeper on the front end, just the transition sleeper, but I was in the LA sleeper on the rear and didn't get up to the front of the train to check.
On days when the rear of the train continues through to LA, there is an additional coach and sleeper on the rear of the train that is uncoupled in San Antonio and attached to the Sunset Limited the morning following arrival in San Antonio.
Photos of Union Pacific Challenger 3985 en route to the Super Bowl in Houston, Jan. 2004