We have a 2014 831RESS and love it, I'm sure that you will too. As for the inflatable mattress in the sofa, I believe that it is a queen size. We only inflated it once to make sure that it held air overnight. The one on our previous tt didn't. Because the mattress folds up into a bag, I don't see any way that you could keep the sheets on it. We would inflate it and make it up at bed time. It's a pain to have to do that, but we don't take extra people along too often.
I hope everything works properly when you pick it up. We were fortunate, have not had a problem....yet. Good luck.
2017 Sunseeker MBS 2400R