Many have made the, "This is brand new, I don't need to worry about leaks." mistake. Regardless of new or used, check and seal the roof edges and around all siding openings (doors, windows, lights, etc.). It seems like a lot, but a bit of preventive maintenance up front is so much easier than repairing the leaks and resulting damage down the road. That applies to the electrical, plumbing, wheel bearings, etc. Go through it thoroughly the first time. Then you can use it for that sought after relaxing.
Mine is a 19 foot box on a 24 foot frame hitch to bumper. Plenty of room for sleeping but no room for moving around inside for 2 adults, 2 kids, and one dog. The cook has to step aside for anybody to use the restroom. I wouldn't want to go any smaller than that.
On edit: I wanted to include that I paid $10k for mine brand new with all the basic amenities inlcuded. That was in 2004. I haven't priced these kind of units since.
Good luck shopping!
