I would add touching every surface of the interior to detect past or current leaks. If there was or is a leak, the area you're pushing on will be soft and give way. Check inside all the cabinets, closets, pass through storage, walls, ceiling, etc. Check the shower, look for cracks in the pan and for soft areas.
Bring a ladder and check the roof. If the roof is dirty, it's probably not been cleaned and/or inspected lately. Look for areas that have been patched.
Hopefully the unit will be hooked up to shore power so you can check the electrical system(s). At the very least, you can use a nightlight to check the outlets in the unit.
There are appliances to check out, depending if the unit is hooked up to the appropriate power source.
I would walk around the unit looking at all the joints and verify they are sealed.
As usual, there is the internet with a numbers of suggestions on what to look for when checking out a used unit.
Based on my experience, buying new has its risks too. My 250RL leaked so badly it went back to the factory for repair. Came back like new and has been leak free ever since.
Good luck and happy camping!