The "hillbilly" AC Mod done on my 13' Scamp
by the prior owner was a oversize AC set on drawer rails set in the bottom of the closet. It seemed a doable project but in this case was bound to fail due to the shoddy work and the materals they used. For one, they used sheetrock screws that were too long and were easily pulled out under stress. Second, they used too large of a AC unit which was too heavy when cantelevered out of the closet. For all I could tell, they would of had to add some sort of outside bracing when the unit was in use, otherwise the unit would of simply tore itself out of drawer rails and rolled out of the trailer. Consequently, they eventually just GLUED the unit into the bottom of the closet, where it eventually burnt up due to not getting needed outside air to the heat exchanger. Like I said, a real "hillbilly" mod.
You can put a ac unit on a slide-out shelf in the bottom of the closet, just take in the fact that you'll need plenty of bracing to support SEVERAL pounds of AC on the sill of whatever opening you'll be making in the side of the trailer. Also, since the AC will be essentially able to slide in and out, you better come up with some sure-fire way to secure the unit when traveling, or else a sharp turn may result in a torn-out hole in the side of the trailer and the AC laying in a roadside ditch.
I modded in a 5000 BTU unit into the closet to replace the monstrosity I had aquired, and with some creative ducting on the back-end of the unit was able to get exhaust AND intake air to share the same single opening with any further modification. I also used a paint
roller drip-pan as my condensation collector and base for the unit. It's on it's third year of almost constant service and it works just fine. I have a video of the mod on youtube.
The AC mod is around halfway into the slideshow.
Good luck on your project.