I believe one of the earlier posters who commented that if you pay PDI, then you simply don't know how to negotiate is right on track. Buying a trailer is like many transactions. You have to consider all the factors that are important to you--before concluding a deal. If having someone explain how the trailer works and making sure that it is thoroughly clean at time of pick up, then it is incumbent upon you to, ideally, place those terms in writing before paying any type of deposit or making a purchase commitment.
If you foolishly make a commitment to buy without covering all pertinent considerations, then you should expect trouble like some of the posters on this topic. Clearly, in a way you can't blame a dealer for attempting to charge someone make ready charges when the customer is too dumb to protect himself. Thus, there is no right or wrong here to argue about, there is only a need to thoughtfully protect yourself. All this talk on this site about whether you got a "wholesale" price or paid top penny means absolutely nothing. Indeed, if I was a dealer, I'd be the first one to charge a very high PDI fee to someone who did pay top penny on the grounds that a stupid guy like that will keep on paying.
In my case, I bought a 2006 Safari at a 20% discount from an out of area dealer without paying a dime of additional fees. I covered all the important considerations in writing prior to making a credit card deposit, even telling the dealer that any talk about supplemental charges or fees would kill the deal. Since the details of the deal were described in exchanged emails, it would have been simple for me to back up the trade with my credit card company in the event the dealer tried to add fees. Funny thing though, when the dealer determines that you know what the hell you're doing, you get a pass. The trouble always gets dumped on the idiots!