Definitely follow the PDI to the letter no matter how long it takes. Refuse to sign any paperwork that completes the transaction until you have completed the PDI to your satisfaction and arrived at a written agreement
to fix any deficiencies.
For us, what worked, was to have my wife check all of the "fit & finish" stuff, ie. cabinets, doors, exterior & interior appearance, etc.. I checked the systems and overall mechanical worthiness.
Just as the PDI says, try everything! When you show up they should have the unit hooked up to water, electricity and (yes) sewer. Run water at all sinks and shower, flush the toilet several times. Then drain all tanks to insure that everything works. The refrigerator should already be on (running on AC) switch it to gas and open the door (to raise the inside temp) see if the gas kicks-in. Try the heater and water heater. Etc., Etc., Etc.
I realize this sounds like overkill but unless you have stumbled on a dealer who sits at the right hand of God than it is good advice to cover yourself. This is much more involved than buying a new car (something most of us have dealt with). You are buying a house on wheels and everything about it needs to be checked to insure its' serviceability. Too many dealers just want your money, will promise you anything, and deliver very little or will stall all repairs forever.
We were lucky! Our 5er had only one small problem and it was fixed before we left the lot (and signed the sales agreement). Our dealer is really very reliable and honest but we learned this over time
, with add-ons after purchase and warranty repairs, not at the outset.
If you are a first time RV'er make sure you clearly understand all the systems and how to use them before you leave the lot. Your dealer should be more than happy to spend the time with you until you are comfortable with everything!
I hope this helps, and again, while this may sound rather anal, it will save you many, many problems in the long run.