Musty smell could be coming from a lot of sources.
Possibilities:
Camper had been closed for a long time (especially in a warm location);
Shower curtains were put away while damp and have mildewed;
There is a water leak in fresh water line somewhere in trailer;
There is some old food in one of the cabinets or drawers;
Something has spilled and dried on the seat cushions;
I'd try leaving the trailer up for a while with the windows open and see if that does any good. We spray 'Febreeze' liberally on the fabric cushions and beds. In fact, we leave a bottle of it in the trailer. If a good airing doesn't solve problem, I'd dry clean, or at least cold wash the cushion covers. This is going to be a process of elimination. Try and do the easiest things first.
Good luck with your TM
Let us know when you find the 'culprit'.
Nick