The PDI is for your benefit. The you should take your time and inspect the trailer from front to back, bottom to top. The unit should be hooked up water and electric. Make sure you can operate everything, stove, fridge, TV's, antena, slides in and out, jacks up and down, hookup to truck, tanks drain and flush commode. The dealer should make any minor adjustments or repairs while you wait. Then I would try to stay on their lot or close by for a day or so to really give it a test. Some folks even take a video camera and record some things.
A very good PDI will take about 3-4 hours. And you should not pay extra for a PDI.
Keep us informed.
Jim M