Get on a ladder, look at the edges of the roof for complete caulk coverage. Do similar with the doors/windows/exterior lights, etc.
Add water to the tank and energize the water pump. Leave it on during the whole PDI. Verify that the water heater comes on. Open cabinets around the sinks and inside/outside showers, and check for plumbing leaks. Look behind the toilet before/after flushing it.
Open all access panels, verify (by touch too) that all the wire nuts and other connections are secure.
Verify that the battery is getting charged while plugged in. This is a good time to wrap a bit of red tape around the positive battery cable for future reference, as it is likely not identified otherwise.
Test the A/C, TV, radio, furnace and other propane appliances.
That's off the top of my head. There are comprehensive PDI lists around here somewhere. A good inspection of the running gear is not a bad idea either. These manufacturers have been known to leave the wheel bearing with just the minimum amount of grease.