I think PDI's have a lot to do with what you buy. For example, if a dealer is selling a $20,000.00 travel trailer, he doesn't have a huge profit margin. I'm guessing on a $20K trailer, they're making somewhere between $800 - $1300. So if they can bang you for another $200 dollars, that helps the deal.
When you buy a $300,000 RV, the dealer is probably making between $4,000 - $8,000. Typically on that larger RV sale, you don't see PDI charges in the documentation or even discussed.
I've ALWAYS done "Out The Door" pricing on everything. I don't want or need to know about what items they might try and charge me for. At the end of the day, I'll pay this amount and that's it. In California, tax and license fees are just about 10%. So I decide what I think the price should be, add 10% and make my offer.