Help with new engine decision

Can't quite describe the smell - it attaches to clothing and anything left in the RV for a while. Always washes out. Had a $100 leak test by useless Camping World in Houghton Lake a few years ago and they gave it A-OK. They said the coach had no window drainage system on the passenger side, an omission from the fractory, no less. Paid them hundreds to fix the problem with no result afterwards. That was my first lesson in RV's and the wonderful world of Camping World/Marcus Lemonis Industries!!!! But, it is now a 21 year old coach, so maybe in its history it had a leak or two. I have personally removed the carpet, sans the doghouse/driver/passenger area and the back bedroom. I replaced with Allure vinyl flooring after a couple of Kilz 2 coats to the underlayment. Helped a lot. But still there is a musty vibe still in play. Tried Febreze, odor bombs, etc... still no progress. Walls and roof seem solid, no dips or bends (except back bedroom corner, but I did have a small leak during Tropical Storm Debbie last year in Florida. But that was ALOT!!!! of rain!

Quite honestly, it smells like a mildew of some sort. You never notice it when in the RV, except when you exit and smell the remnants much later on your clothes. The coach is supposed to have an aluminum wall structure, as well as the roof. Maybe there are bits of plywood holding their secrets. But, this is getting off-topic of the original post. Thanks for all the great replies so far!! Jamie