You already hit my top one - leaks, whether it's structural or plumbing.
I get asked to go with friends a lot when they're RV shopping. I make sure they tell the seller ahead of time to fire up everything in boondock mode - fridge cold on gas, hot water, etc.
I also bring a voltmeter to check the batteries.
Since you're looking at a 5'er, check the inside/backside for the rims for wheel grease splotches, looking for bad axle seals.
Look for small stress cracks in the siding from excessive frame flex, especially around the slide out corners.
Check the rubber boots sealing the slide outs for age cracking or tears.
I'm sure I'll think of more...
