We have owned 3 class C motorhomes, by no means does that make me an expert but I have learned a few things. Starting on the outside, first thing is to look at the tires. If you find so much as one weather crack you need to automatically ask for 2000 off the asking price. I will not haul my family around on weathered tires. Your looking at around 1500 for a good set of tires and you need to have the alignment checked while they have it, that's another couple hundred. If they just replaced the tires with a brand you can't pronounce, I would still say you want to replace them. I tried the cheap route once and the motorhome never handled right because of the weak sidewalls.
This is my own opinion, but I would not own one that doesn't have air bags. When we bought our Sunseeker I made the guy knock off 700 to cover the cost of installing them. They make a huge difference in the wind and for towing.
No matter how good they say the awning is, make them pull it out and show you! If it is cracked and weathered that is another major expense.
Look really good for any damage around windows and any seams that may cause leaks or may have already allowed water in. GET ON THE ROOF! If they tell you that you can't walk on the roof just walk away. You can walk on these rubber roofs. Look for cracked vents and tears in the rubber roofing. Lots of people snag tree branches and they never know it caused a tear.
All of mine have been under 15000 miles so I have never been worried about the engine and drive train. If there is a problem in that area you should notice when you drive it.
On the inside I would just look for any brown spots on the walls and ceiling. Check over everything really well and look for any abnormal wear and tear. If it has slide outs make them put them out.
When your driving it pay close attention to any popping sounds when you are turning corners, that is a good indication the ball joints are going out, this is a bigger problem with the Ford chassis. Also take note on if it sits level. If the rear is sagging a lot that's an indication they may have towed a lot or over loaded the motorhome a bunch. Just remember that most motorhomes are fairly loud going done the road, they make noises you may not be used to, doesn't necessarily mean its bad. Any cluncking, popping and excessive squeaking from the outside would be worth further investigation.
I did this on my phone so I apologize if its full of misspelled words.