To the OP (Original Poster):
There are a lot of people on this forum that think that if you can't go in a sleeper, it's not worth going on a long distance train at all. I am now one of them -- but hey, I am also now within shouting distance of 65 years old! My first long distance train trip was twenty years ago, when I was in my mid-forties. Hubby and I went coach from Pasco to Boston. We went two days and nights, then spent a night in Chicago with the aunt and uncle, then one more to Boston. On the way back, we went two days, then spent a night at the Isaac Walton Inn in Montana, then one more back to Pasco. I packed tote bags full of food (there is water on the train, but I always make sure of a good supply by bringing bottles like I use for hiking). We slept OK, not great. We had an absolutely wonderful time on the train.
I am still willing to travel coach on a long day trip. Due to budget constraints, and lack of enough points, last November I got up at 4am and moved out of my comfy roomette (it was re-booked from the same station stop, or they probably would have let me sleep), and moved into coach on the Cardinal. Yes, I brought my own food for that day (tuna and crackers, dried fruit, nuts, chocolate etc). Depending on the level of gluten-free that you need, eating once in the dining car is another wonderful experience. If you just need to "avoid" gluten, that person can undoubtedly eat a steak and baked potato (request that any sauce be left off, or served on the side). If it's celiac disease, take no chances and bring all your own food.
Don't let anyone discourage you just because a sleeper is not in your budget! If you can't afford it, you can't, but the train trip itself is an experience not to be missed. Make sure you spend plenty of time in the Sightseer Lounge car. And we'd love it if you report back to us on how it went. Have a wonderful trip!