First, get one with complete maintenance records. Make sure any maintenance records you get are complete. An oil change once per year is far from adequate.
Get one that has been well taken care of my knowledgeable owners.
Watch out for the paint checking issue in those years. I would prefer one that was kept under cover.
Get it inspected by an independent shop.
Floor plan is important.
Stay with the better brands: Newmar, County Coach, Tiffin.
We are fine with our rear radiator. The issues we had were due to being out in the sun and incomplete maintenance records.