Although your motorhome is a vehicle, the quantity built versus cars, trucks and SUVs is miniscule. As such they are assembled manually, not with robots. Motorhomes are very complex. They have 12 and 120 volt electrical systems, city water and onboard tanks, propane and heat pump heaters and electronics galore. It's a car and a house and the walls move in and out. And when you drive it, it is like an earthquake shaking your house. The PDI is needed to find out what works and what doesn't. The factory will check it, your dealer should check it, and you must check it.
Factory pickup with manufacturer PDI: With many motorhomes, you can have the option of picking it up at the factory.They will charge for this. With factory pickup, they know they must fix it before you take delivery. With dealer pickup, the manufacturer may leave small items for the dealer to fix. Therefore the theory is that with factory pickup they will be more careful to build it right. Is this true? We don't know. We were going to have a factory pickup but plans had to change and we picked up at our dealer.
Dealer PDI when picking up from dealer: Your dealer should do a PDI before you arrive. Our dealer did a good PDI and we found only minor imperfections that they fixed quickly.
Your own PDI: We found a very extensive PDI list on the Internet. It was our intention to spend days at the dealer and check everything. As it turned out, it rained heavily everyday we were there. We checked what we could from the inside and headed out. We stayed within 100 miles for a few days. Better to stay close to your dealer for a few days to a few weeks. We did not, but were totally willing to return (350 miles) for any repairs.
Understanding Warranties: This is tough, but read the warranty. Most manufacturers give you one year "bumper to bumper". After that you still have warranties on appliances, awnings, engine, transmission and other items (see your manuals).
Extended Warranties: We did not get one. We have never purchased extended warranties. If you think you want one, talk with others who have purchased them and see what their experience has been. We have heard good and bad, but can't help you here.