Sorry to hear of the issues.
First advice, trust no one and learn from this experience.
Second advice, be firm, professional with the owner/manager. Feel free to throw in the inspection vs mold vs health/legal ramifications of that.
Third, stand your ground and pray they play ball with you.
Fourth, consider the first advice again and use it going forward. You got a bad deal that may be made better or may not.
May seeem like a lot of money, but a little roof sealing and some cleanup sounds like all it needs based on what you've presented. Be resourceful, don't be the victim.
Fifth advice, not unlike any other man made item, rvs need maintenance, a lot of it, and can be or become expensive. IMO they are NOT a good alternative to residential construction except in certain circumstances.