This may (or may not) be the dealer that I have bought three trailers from. They are top notch in every way and if they cannibalized a part it was to meet a need and take care of a customer. That is the kind of service I have received from them in the past and they pride their dealership in doing whatever it takes to take care of the customer who is in front of them.
I think that it would pain a dealer to pirate a part off of a $30k camper knowing that it is now effectively out of circulation until the parts can come in. But if it meets the needs of a customer who has invested a considerable amount of money with you already, well, the customer standing there wins every time compared to the customer who may come in sometime in the future and may buy a camper. In business, you always take the sure thing, not the gamble.
In a perfect world, everything would work perfectly and never need repair, parts would always be plentiful and no one would have to worry about having a sore P-______. But we live in a world that is not all gum drops and lollipops. I'm sure that if it was your camper that needed the cannibalized part, there would be no hesitation in telling the dealer, "Heck, ya! Take that part and give it to me!" That is just the way the world works.
Let's cut the dealers some slack. Not all of them are bad. Some are. But not all of them. Throwing them all under the bus is just overkill. Rant over. Besides if the parts are put back and all is good at the PDI, who really cares?