I 'm surprised your Coachman didn't come with two, well maybe not
Anyway, you have a minimum 45amp charger/converter, so that will handle two fine. If you do, get the same battery you have now. You don't want to mix battery kinds (deep cycle, marine, starting) or different size/capacities. If your coach wasn't new, I would suggest 2 new batteries. You don't want to mix old and new either. When you have 2 or more batteries on the same bank, you want replace them all at the same time.