I have mine in the tongue box and it works great. This is what I did. Cheap 5000BTU AC unit. I bolted it to the front wall of the tear. Just as if it was being installed into a window at home. About 6" sticks into the interior of the teardrop. The rest sticks into the tongue where it exhausts. Inside I have a headboard the is about 8" deep on the bottom and 4" at the top. The angle makes it nice to sit up and lean against . In the picture there is a vent panel for the intake. I built a wooden Duct that deflects the cool air up and along the roof so it does not blow on you. It works great. It ran every night at the CRA outing without any problems. You do have to prop the tongue box lid open for the exhaust. I water proofed the inside of the box so no worries there.
Here is a shot of the box packed for travel. There is enough room to get my chair in between the AC and the front of the box. The AC can be unbolted and removed without taking everything apart.
Here is during construction.
Here it is working.
