Tires 1500, Brakes, foot hitting floor? Master cylinder? 500 ish, new rotors calipers, new rear shoes maybe another 1500 more or less, (although they could be fine, calipers etc) Could just be the master cylinder. Gas leak, I have no knowledge, could just be a piece of rubber hose somewhere
Rebuild walls? probably not worth it. Thing in great condition only worth 9, 10 k. I would stop the leaks as best I could, bleach the mold, and just drive the thing. Or option two, sell it for 5k let the next guy deal with it. ( you would have to a least make it safe though, in my opinion.
Sounds like you are at an RV dealer, get away from them, take it to your local mechanic