Welcome to the forum.
Check for evidence of leaks, a small leak can cause a lot of damage if not fixed quickly.
Anything you look at, check the age of the tires. RV tires usually age out before they wear out. Many buy new tires when they reach 5-7 years old.
It goes by the manufacture date, not the date they were installed. If the tires are old, factor New tires into your price negotiations.
With any used RV, condition is the important factor. A low end unit in good condition is usually a better buy than a high end unit in poor condition.
If the interior is dirty and not maintained, it's a good indication the engine an mechanicals were not service properly.
There's a lot more but, this will get you started.