PDI is what car dealers call prep, it is done by a service technician before the buyer arrives to pick up the RV. What the service tech or salesman does with the buyer is a walk-thru.
Whether it is added to the contract as a line item or it is included in the price, the buyer pays for it either way. It is only the final out-the-door price that matters.
A dealer can use things such as PDI and title fees as a line item in order to keep the advertised price lower. Like I said, it is only the out-the-door price that matters.