In all likelihood you'll get what you pay for. Case in point. I know two people that bought new RV's this year. Both were totally new to RV type camping. In both cases the did not pay a Dealer Prep/PDI fee. And that's exactly what they got. Very minimalistic prep and next to nothing PDI. Consequently, they new next to nothing about how to run or operate their systems. One guy never knew you should jack up the rear of the truck before you try to put on or take off the WD bars. His cheater bar almost took his head off when he was trying to release the bars. The other spent the best part of two days messing with his hot water heater trying to get it to heat water before someone told him it wouldn't operate at all if he had it on both Propane and 120V at the same time. In both cases, being absolute newbies and no familiarity with RV systems - paying $200 or $300 for a comprehensive PDI would have save them a lot of time and grief.
If you are comfortable with very minimal dealer prep and PDI fine, if not, consider what you may be a wise investment to save a time and grief down the road. If you expect you'll get it and not pay for it one way or another you're dellusional. No one size fits all answer.