I have gone coast to coast many times in coach. I have even done so on bus seats which are not nearly as roomy, but I am blessed with the ability to sleep anywhere, so perhaps I am not a good reference. I can go to sleep on a hard cement floor, with the sun at high noon, using only my arm as a pillow.
That said, I offer these pointers. First of all, don't do what most folks do when they board. That is immediately on boarding, recline the seat and try to sleep, or even at 10PM when the lights are dimmed on most overnite coaches. Instead, try to stay awake as long as you can...until your eyes won't stay open, and then try to go to sleep. I think that you will feel better if you don't take a series of short naps, and instead try to sleep for a longer period. If you can't sleep, try reading for a while, or take a walk up and down the train. It is no good to just lie there if you cannot fall asleep.
I don't recommend trying to sleep across two seats. You will probably wake up with some serious neck and/or back pain. If you can get two seats, sleep in the window and recline it fully. Sleep on your side, turned towards the outer seat and leave it reclined less, so that you can rest your head in a corner created by your headrest and the side of the other seat headrest. i like to sleep curled up with my knee on the cushion, and my other leg crossed over my foot.
If I can only get a single seat, I just recline, and sleep on my back, with my legs extended as far under the seat in front as I can reach.
Whatever, you find comfortable is the best for you. Also you will find that on a two or longer night journey, you will definetly sleep better after the first night.
Good Luck!