Negotiate a out the door price to keep those "add on prices" down and let them know you want to see everything working before you sign anything. Our walk through took about 7 hours and I saw everything working, the fridge had been on overnight and working on propane and 120 volt, AC operation, and AC inspection on the roof with the shroud off, water heater on propane and 120 volt, removal and inspection of the anode, microwave, stove, oven, water fill, tank drain, holding tanks full and then dumped, landing legs operated down and up, TV's and stereo, shower, sinks, slides in and out, roof inspection, all ceiling fans, the trailer vacuum, tested all 120 volt outlets, generator checked under full load for voltage, amp load, and frequency, transfer switch test, awning operation, in and out, and towed the fifth wheel to check the brake operation.
I gave the salesman a list of what I want to check and see operate before hand and that I would be there on the PDI date when they opened with my own lunch, and if it didn't work or operate completely the sale would be off.
The tech that ran through the 5'er said they had never been through a unit so thoroughly before and said that the salesman had told him to make sure everything ran smoothly because I was a picky SOB.
Just remember "you are the boss and they need to keep you happy" and if you schedule the walk through PDI close to the end of the month, it will help also.
The information is out there, all you have to do is let it in.