Well,you're looking for a 5'er,so you're off to the right start.
These pull way better than bumper pullers.
If you're looking for a used one -just use the basics:
Look down both sides-make sure you see no warping-meaning water leakage.
Get up on the roof and check for tears,dents,holes,etc.
See how much caulking/sealing is visible.
Check inside of all cabinets mounted to ceiling that there's no squish when you push on any wood/whatever-meaning rot.
Test the a/c-if equipped.
Before you go to look at one,ask if propane bottles have enough to test heat and water heater-make sure all appliances work.
If you have the hitch already installed,ask for test tow,that way you can also check brakes and lighting status.
Check tires for wear and dry-rot.
Hope this helps!