Look at the tires and get the codes off them to make sure how old they are.
Make sure the generator works and powers everything.
Make sure the aqua hot works.
Make sure all the air conditioners/heat pumps work.
Naturally check the fridge and cooktop.
Get up on the roof and check it. Check the inside closely for water damage.
Look the cummins over top and bottom, make sure no oil leaks. Allison tranny too.
Have the seller show you the jacks work, have him give you a look at the generator and how it slides in and out. Have him take you for a ride and listen closely. This will also help you determine if brakes and airbags are good. Which reminds me make sure it's level so you know one of the air bags isn't leaking.
A reputable inspector looking at it might be a good idea, it's a big purchase.