OK, it isn’t really an installation, it is more of a “connection”.
I bought a “PETCool ThermASSURE 2500”. It also heats. It is made for cooling dog houses and air is delivered through a 2 inch spiral wound hose, with a return hose the same size. The pet cool is self contained and weatherproof.
Here is the deal. I didn’t want to punch holes in the skin of the trailer. I didn’t want to give up any of the galley either. I had tried to get a roof AC installed by a local dealer, but they wouldn’t do it without a release from the manufacturer. I am glad I did it this way.
First I had to decide how I was going to get the air into the inside, without cutting holes in the aluminum. I saw that there were essentially three openings available. The door, the window and the roof vent. Since the PETCool is “free standing” and has an approximate 18X15 footprint. And coincidentally, there is a nice flat spot on the roof, I opted for the roof vent entrance. A quick trip to the parts store and I had a new vent cover for my Fantastic roof vent.
Now for the work. The air flanges for the PETCool are about 2.5X5 in and have 4 holes each, at the corners. I stopped at the hardware store and got 8 M8X3/4 in machine screws, with nuts. I also got some nice big washers, for spreading the load over the plastic, and two 2 inch plastic PVC caps (a snug slip fit over the air flanges). I also got a tube of clear silicone caulk.
I examined the structure of the vent cover and decided on a front/back arrangement, since there were some structural/mechanical feature that might have interfered with the operation of both the vent cover and the A/C had I mounted the flanges left and right. I took the hole saw, which was supplied with the PETCool and sawed two holes on the centerline of the vent cover. I placed these holes as far apart as possible. I was contemplating putting the flanges on the outside and had cut some wedge shaped spacers. On further analysis, I decided to mount the flanges on the inside, with the air supply tubes passing through the vent cover and the flange plates on the inside. This does a better job of spreading the stresses in the plastic.
I carefully measured and drilled the holes.
Next, I put a good bead of silicone around the screw holes in the vent cover, and around the base of the air tubes on the flanges.
I inserted the flanges, one at a time into the holes and snugged down the screws just over finger tight.
It looked pretty good at this point so I just let it all set and dry over night.
While it was drying, I made a small modification to the PVC caps. I drilled a 1/8 in hole about a quarter inch from the open edge of each one and started a small self tapping screw.
After it was all dry, I reinstalled the vent cover and fastened it down.
Now, the vent is waterproof, the screws hold the caps on to weatherproof the vent cover. When we arrive at the camp site, we simply set up the trailer, set the AC on the roof, loosen the screws and remove the caps, pop on the hoses, plug it in and turn it on. It works pretty darned well. It Cools the cozy cruiser down to very comfy in a short time. When it is over and time to leave, I just turn it off, pop off the hoses, put the caps on, barely tighten the screws (they act like grub screws and just keep the caps snug). Since there is still some water in the AC unit, I just set the whole thing in a plastic box, for transport.
One more thing, to make certain of the direction of the condensation run-off, I inserted a clear plastic hose (once again a snug press in fit) into the drip hole, to direct the condensate to the front of the trailer.
I am over sixty, and the PETCool is small enough for me, even with MY back to put on the roof and take back down. I think that a 5,000 BTU would have turned the little space into a meat locker.

After I have given it a "real world" trial, I will update with any modifications. I already am looking for slightly longer hoses, and am wondering about the efficacy of insulating the cooling side hose.
I just posted a picture of the roof vent with the Petcool air flanges installed and the caps in place. This coming Sunday we are taking it out for the first "real world" trial. I will post more pictures and a report of what works and what could be better when we get back.