Since I have concluded I have to stick with a BP trailer, I'm wondering about adding AC to one.  I know I've seen one 4 Star for sale with an AC unit, but do I have to buy a trailer that has been specially framed or can I just add AC to any of them?

The air conditioner normally replaces the roof vent.  The trailer must have the required framing to support the weight of the unit.  The bare roof structure is not sufficient to support the air conditioner. 

The difficult part is that you have to get the unit up, into the center of the trailer and square in the opening without damaging the roof or the gasket on the bottom of the unit.  After that it is not difficult to complete.