Hello folks and welcome to The RV Forum.
Naturally, you will want a mechanic to check out the vehicle portion of the rig like brakes, transmission, etc. As indicated by Robert, leaks can be a big issue. They manifest either as yellow spots on the ceilings and walls or as delamination (bulges in the outside fiberglass). Leaks most often occur around opening like windows, vents, etc. and anywhere two surfaces join. If it has jacks, do they work?
The refrigerator can be a costly replacement. Unfortunately, they take a long time to cool down. So call the day before you go and ask them to turn it on so that it will be cold when you arrive.
Make sure the A/C, water heater, and generator all work. Also set on all of the furniture. MH mileage isn't necessarily an indication of how much use the rig has had. Are the seat cushions still comfortable?
Check out our library. There is a lot of good information to be found there. If you have questions for which the answer is in the library, just post here and we will get back to you.
Welcome aboard!