My biggest mistake was not testing the gas water heater. Spiderwebs in it caused massive amounts of soot up the sidewall later in my driveway. Turn everything on including the water pump (to check plumbing)
Also consider having them install paper towel holder, coat hangers and bathroom vent covers while you are taking delivery. I wanted them to put the screws in not me (i.e. Screwing into something I shouldn't have)
Finally if you're picking up a new trailer make sure and take it to the tire place and get your tires balanced.
2016 3500 6.7L DRW
2010 Starcraft 329BHU TT