If what you smell is just OLD, then air it out and prime with something like KILZ. Then paint. If what you smell is MOLD, then that must come out and you start over. If painting works, you are good to go. If painting does not work, then you can cover walls with something and go with that. Tackle it in stages, and stop when something works. That's my story and I'm sticking to it.
1993 Gulfstream Sun Voyager DP 30'
200 HP Cummins B5.9, 4sp Allison AT542
Toad 1998 Honda CRV, 2006 Honda CRV
Life's too short to drink cheap beer