There are a slew of PDI lists to assist you. You can do a general internet search, every RV forum out there and specific RV group seems to have them, and you can also purchase them online. Ranges in length from a page to a small book.
Commonly, a three pager at many dealerships, but always possible to just sign it off and not do it.
Of course, ones knowledge and tools limits ones ability to do some of the PDI steps similarly to doing repairs and maintenance. That can be overcome, of course, with some paid assistance.
Time and money at this stage is probably the best thing a buyer can spend. Think days, not hours.
The RV forums have tons of problems suffered by owners that were otherwise avoidable.
One of the worst was a buyer of a used diesel pusher who was, of course, in a hurry to load up and go on a trip. So he didn't "have" the time to pull any fluid samples, which had he done so would have flagged a serious developing problem with the diesel engine. During that first trip the engine blew with a $30k price tag of which the dealership helped with but only 1/3 of the cost.
As to new, also plenty of horrendous problems suffered by a lot of folks.
I like to think of an RV being only as good as its inspection, both new and used.
In general, I like to start with what most people never look at, the roof, underneath the chassis, the electrical system, water damage evidence, and fluid pulls if motorized, both new and used. Assume tires need to be changed and all maintenance brought up. If bought used automatically replace the toilet and bed. Evaluate and probably change the batteries.
Note: it is not uncommon for a dealership to not keep up the maintenance of an RV on their lot, both new and used. Check last years model carefully.