Hello and Welcome!
There is a pretty good pdi list in the document's section. Print it out and take it with you.
My personal advice is if you do not understand how something works tell them to SHOW you how it works. If you can get on the roof look at its condition - caulking around the vents - check the AC fins for damage.
Inside look in all the cabinets with a flashlight especially the ceiling for any signs of a leak.
Another thing ask them to put water in the tank before you get there. Then while you are there turn on the pump let it come up to pressure and it will turn it self off then while you do the rest of the inspection listen for the pump to turn on and off quickly. This would indicate a small leak in the water system.
Also have them turn on the refrigerator at least a full day before you arrive as it will take 24 hours to come to temp.
Make sure to roll out the awning and if its power it should operate smoothly in both directions. check the fabric for signs of mold.
These are just a few things to check. Others will chime in with more.
Good lick with your purchase. Happy Camping!!