One of my close friends has a DuneChaser, and has owned it for close to 10 years, he bought it new. It is, I believe, about 40' overall, has the one slide in the bedroom.
In my opinion, it is a well built coach, they have had almost zero problems with it and have towed it at least 30,000 miles over the years. They hauled a combo of 3-4 quads or 2 quads and a side by side, but now just haul the side by side.
I looked for a clean used DuneChaser for over a year and gave up, we bought a 2014 Eclipse Attitude and we are happy with it.
Just go over the seals on the coach, do the usual stuff you'd do to any used vehicle...service the generator and the wheel bearings and brakes, etc. If it hasn't been abused I think you'll find it is a good unit. I know for s fact they are well insulated, he uses about 50% of the propane for heating that his son uses in a comparable sized Tahoe Transport.
I'd not worry too much about the 15's...I'd put Light truck tires on it were it me, but that is something you need to research yourself. I drove exactly 50 miles with the Towmax tires my rig came with before getting them replaced. I've never heard of a 5050 lb axle, I'd suspect they're 6k rated.