Turn on everything on, operate everything to make sure it works. Start with the big ticket items and work down:
1. Generator, does it start and does everything work off of it
2. Air conditioners, are they cold
3. Dash air - is it cold
4. When was the last time the radiator & cooling system were flushed
5. When was the last time the transmission was flushed and looked at
6. Does the 12V water system work
7. Does the refer and freezer work well on 110VAC and 12V/propane
8. Does the TV antenna and system work
9. Look for water leaks
10. How old are the tires
11. Run the furnace
12. How old are the starting batteries
13. How old are the house batteries, what's the water level and are the terminals clean. If the battery water level is down to the plate or the terminals have lot of corrosion, then the owner didn't take very good care of it. There could well be other things wrong, but this is a good quick simple test.
14. Unless they could be frozen, run the awnings in and out to make sure they work and to see what condition they're in.
After that there's still a million little things, but the above are the high dollar items.
Bill