In our camping group, there have been a few people who have purchased and used the PetCool system. What I have seen and heard about them, them are nice all weather systems.
However, problems are that they are really pushing it for trailers. They are good for about 350 cu ft. The Shasta Compact is about 350 cu ft. Anything bigger and the Petcool won't be able to be efficient.
They are perfect for teardrop campers. Less then 140 cu ft.
I think Ross's plan to install a window unit on the floor and vented through the floor will be best, but in case it's beyond my woodworking skill level (which is pretty much piggy breadboards ) and my handyman left town, I wondered if it might be an option. It would involve cutting two holes in the trailer, though, I see.