Hey Mattnday - welcome to the forum! If you get a chance, come on over to the new member area and introduce yourself.
Here is a link to a popular checklist. It is mostly for new, but works fine for used as well --
RV Checklist
In my opinion the biggest thing to check is the condition of the roof and if there have been any signs of leaks or water damage. Those can cause significant damage if left unattended to.
Good luck and let us know what you end up doing.