The A/C would mount in the hole in place of that vent. The framing is already there for it, as is the wiring. The little junction box with the wall plate on the ceiling next to the vent there contains the wiring for an A/C unit.

Ah ha! I thought that what the covered outlet was for. Go me! ha ha I can probably find the right size A/C at Camping World or some place similar right?

Absolutely, I think all roof A/C units use a standard size opening, which is something like 10"x10". You will have options such as ducted/non-ducted, capacity, and heat pump/no heat pump. You obviously don't have roof ducts, so you need the non-ducted (which would be the case for such a short trailer anyway).You can certainly get away with a small capacity because the trailer is so small!

That true and I would not need to extra cost of a heat pump either because I have heater. I just have to make sure it works. Although, I given the small size, I can easily use a small electrical heater I already own and have used before when I was working in a small close size storage area at my photography school when students would check out (loaned) equipment.