When we shopped for our used Class C we did the following:
We turned over the engine and listened to it for at least 15 minutes. Checked the oil and transmission fluid. Drove it at varying speeds to check the transmission shifting.
Checked the tires for age.
Looked for evidence of leaks. Ran the furnace and A/C. Checked the water pump. Ran the fridge.
After we bought it we upgraded the electrical system with a new board. We put a flexible coating on the roof (looks like white bedliner) that lasts 5 - 10 years.
It's 26 feet long. The 450 Ford engine had 65K miles and still runs great.
If we buy another we want one that's even shorter because we spend most of our time outside the RV and only need it for sleeping, and eating in inclement weather. We do most of our cooking outside unless it's raining.