This is the time you want to check everything. Turn on every switch, every light and make sure everything is working. If the rig has a generator start it up and be sure it is powering the outlets correctly. Run the microwave, turn on the water heater and furnace and make sure they fire up on propane and work. Light the stove burners. If the rig has levelers make sure you know how they work and test them out. Look for obvious problems in fit and finish. Turn on the shower, flush the toilet. Open every door and drawer and make sure they open smoothly and latch securely inside and out. If it has an entertainment system, take a DVD and CD with you, pop them in and test that out too. Turn on the fridge and make sure it will work on propane and AC power. Turn on the air conditioning and see that it gets cold. If you find problems, note them and make sure the dealer fixes them before you leave if possible.
Congratulations on your new rig. Enjoy it and have fun.