This would be a good place to start, but I expect you will have to back off that position to reach an acceptable settlement. After all, Amtrak did get you to your chosen destination, and you possibly availed yourself of the meals included in your sleeper fare. And, assuming the coaches weren't 100% full, you could have gotten your AC there, as well as in the sightseeing car during opening hours. Admittedly that's not what you paid for or expected, but it could have been worse.
I would suggest you set your bottom line (confidentially) at a certain percentage of the roomette upcharge. Keep in mind the three P's: patience, persistence. and politeness. And, if your first telephone conversation with a real customer relations person is unsatisfactory, switch your communications method to email and address it to a real person like a supervisor. Also, IMO, you just have to state the facts without too much emoting about the suffering it caused. They are well aware of the consequences of no AC on a crowded train.
Cash or voucher? That's a personal preference. If they offer only a voucher, be sure you understand all its T&Cs before accepting it.
Good luck!