I thought I'd offer some of my experience as an over 2 year full-timer aboard a CLASS C.
#1 - Choose a LAZY DAZE brand RV, I would highly recommend them. (we have one, love it, high quality build, attention to detail etc.)
Regarding Water & Leaks. I've never known a Class C NOT to have a leak in the front window (grrr). Ours did (it's a 1988), we recaulked the seams and have been good ever since. BUT- for your further inspections, press your fingers around the corners of the windows, vents, and ceiling. If it presses in, you have signs of water damaged underneath the surface.
Tires - A very good investment. Find out the age from the previous owner. Try and see the DATE of purchase, that will let you know how old they are. Ours had good tread but had been sitting, resulting in cracking/checking of the rubber. Ended up blowing 2 tires, one after the other. Causing major under carriage and interior damage. Scary & costly. Not to mention being on the side of the road in a town we're unfamiliar with. If they appear old, even with ok tread, on a 1980's/90's rv with low mileage... Tires are likely to be OLD.
Good luck and enjoy your travels!