Obviously, make sure everything works. Hit every switch, lights and otherwise. Fire up the AC and heater, generator (if you have one), microwave, stove, oven, run your water (city and pump), try your level checks, etc...
Check your refrigerator on AC and gas.
But also, check things like:
window shades (are they lose?)
any seams (check for separation, cracks, water/moisture, etc...)
open and make sure all cabinets close and latch
convert any convertible furniture (ex: couch that turns in to a bed)
Before you leave the lot:
check lug torque
check tire pressure
I'm sure there's more that I haven't though of.
hookup and check the trailer running lights before pulling off the lot.