Yeah, what he ^^^^^ said!
Check and make sure all faucets work, the furnace kicks on as well as the a/c, open and close all the slides (if any). Open and close all storage areas as well as work the locks, open and close all drawers and cabinets/wardrobes. Turn on/off all the lights, light the stove and oven (probably the only time oven will be lit) and make sure there are no propane leaks. Show how to use the TV and any dvd players, as well as the difference between the over-the-air antenna and the satellite feature.
Have them show you the converter and go in detail the difference between the 12v side and the 110v side - how to tell "quickly" if you blow a fuse or if a circuit breaker trips.
They should show you exactly how to hook up any tanks to be emptied and have them demonstrate the black tank flush.
These are just a few if what I consider the major issues most people will run across.
HERE is a PDI Checklist you can download. One other suggestion is to take a video (camera/cellphone) of the PDI.
Ultimately,