Those are just modified window units. Or at least, they should be. Make your own out if a window AC if you really want that form factor. The critical thing is that there are two vent lines. The device has to have return air to re-cool. Portable air conditioners are terrible because they dicharge exhaust air out of the area to be cooled. This draws in new warm air from outside however it can find its way in. You can find a 5kbtu AC unit on craigslist for $50 or less and add your own insulated tubing to the front. This runs to connections you can make through your trailers floor and bingo, removable AC that makes no visual difference in the trailer, and you dont have to bring it along when you know you won't need it.
The readily available dog house units that I found are far less efficient than most wibdo units, and honestly I dont think sub 5,000 BTU would be enough given the losses involved with the tubing. I too found those when searching for window units LESS than 5KBTU for my 16 foot. This is because I replaced all of the internal bubble wrap with 1/2" R5 rated polyurethane foam board. I run my AC on solar
so I am always looking for efficiency.
I plan on building or buying a mini split heat pump in the future. I would be tickled to find an inverter driven 5k window heat pump though!