I should have mentioned that the trailer manufacturer (Grand Design) claims that the trailer is well insulated. They state that it has R-40 in the roof, R-11 in the floor and R-9 in the walls in their brochure. The manufacturer also claims that the 15K BTU AC unit should work just fine and they have not gotten any complaints.
As for adding a second AC unit I was just curious and I was mainly interested in the actual AC unit install on the roof itself and all that goes with that, like sealing the roof and securing the AC unit its self. I wasn't that worried about the electrical aspect of the install and I don't mean to minimize that aspect of the install, its just that I'm an engineer and I've worked with electricity (AC and DC) my entire career, so I'm sure I could figure that out. All that said, I honestly don't think I'd ever do it. Seems rather crazy to plan on adding a second AC unit to brand new trailer. It would make more sense just to buy another trailer if the 15K BTU unit can't do the job.