You do your own PDI while you are still looking at it on the sale lot. (Realize if this is a slim sale-the dealer may or may not fix what you find).
----You should look closely at trim pieces for fit/finish. Walls for bubbles/tears/ETC, Upholstery for stains/rips tears/discolor/ETC, open/shut ALL cabinets, drawers, doors - checking for smoothness of operation and how tight do the catches work. Open/close ALL windows AND blinds. -- Check mattress (especially if used unit) for excessive stains or cuts/tears. -- Look at flooring and ceiling for anything (water marks, tears, ETC).
----Go outside, look at the outermost corners of the BOTTOM of the sliderooms -- check for softness (which means water damage). Peak under the unit for excessive rust, oil leaks, ETC. Check tires for wear, age. Look at the INSIDE of the wheels for grease (possible wheel seal leak which could lead to many problems). Look at the roof if possible around all the seals.
----Now ask salesman to demonstrate operation of sliderooms. -- Sounds like a hassle, but you should be able to do all this WHILE looking at the unit. Likely it will consume 30 minutes. A GOOD salesman will be willing to allow you to do these things. (If he is not hiding something).
IF you find ANYTHING, Make a list of expected repairs and make sure the salesman writes up a proper "WE OWE". Both parties are to sign it. --- REMEMBER, if it is used and more than 3 or 4 years old. The dealer likely wont fix every single flaw. Some things do get "used" faster than other things, so do be TOO critical over small scratches, tears, ETC. I mean "Is it NORMAL usage, or EXCESSIVE damage"?
Give the dealer ample time to complete repairs and perform their own PDI. -- Now on delivery day. Do a quick check to insure your "hit list" has been addressed, then make sure you are shown & demonstrated how everything operates. This includes. Make sure the fridge is cold, water is hot, A/C works, furnace works, stove/oven works. -- Consider temps with furnace-A/C.