I am new to RV myself. In fact I am still looking for one. But I would be very upset at a dealer who cannot diagnose a problem in a reasonable time. After all thats what they do. A $1,000 bill for trying to figure out what is wrong. I would go talk to these people and let them know you are upset at how they are treating you.
You take your car to a dealer and they give you an estimate up front or if it exceeds the time they thought they call you and see what you want to do. To take your vehicle in and they have a free ride to bill unlimited amounts seems to smack of unlimited fraud.
Course that is just my opinion.
Mike