The live in it for a day or two is a must if at all possible. If the dealer can not accommodate this on site ask them to arrange it at local campground etc..
Besides what has been said above.
Make sure they burn in the furnace at least a few hours twice over a two day period. This is a big one. If not you will spend two days in a smoke filled trailer the first time you need to use it.
This time of year they ship the trailers winterized and the dealers don't like to put fluids in. This you must insist on and then they need to re-winterize, if you live in a cold climate or will be traveling in the cold have them install the winterizing kit or you can also look at http://www.airforums.com/forums/f516...-a-127922.html
Have them walk you through a complete winterize including the hot water heater bypass and drain.
Make sure they can connect the trailer to 50A power and run both zones in AC and HP modes. Check for function and vibrations/rattles.
Run the furnace and check inside and out for vibrations/rattles.
Check the UL carefully for stress cracks where drawn tight.
Look at all the cubby holes for bad interior trim or edge banding. It will undoubtedly need repairing.
Take a DVD with you for them to play in the player. Make sure all 3 TVs work and sound comes through the overhead speakers.
Run all the appliances in each mode (i.e. fridge on electric and gas, hot water heater electric and gas).
Look behind the toilet, sink, in all the cabinets everywhere you can for leaks and plumbing pinch/rub points etc..
Check the exterior stove vent as the plastic keeper arms came out the first time tried to use it. I have ordered a stainless steel replacement.
Look at the windows carefully inside and out for proper installation. I have seen a couple that were flat wrong.
Almost forgot, take pictures of the stupid stickers in the shower and have the dealer remove them. Also make sure the bottom of the shower looks new as mine was very dirty from others stepping into it with their shoes on. It still does not look that good.
The more time you spend in it and operating stuff the better off you will be.
Better to have them make all corrections before accepting it.