The PDI or prep should be done by a service technician before the customer arrives. A proper PDI can take many hours, as many as eight hours.
The service tech should fill all tanks, and check operation of all systems and appliances. If anything is incorrect the manufacturer is contacted, a warranty claim made, and warranty repairs accomplished.
After the service tech has verified that every aspect of the RV is correct, adjusted, and working correctly, and then the customer arrives and does the "walk through".
If the dealer does not perform this function then the customer is being short changed.