A 23 foot RV is going to have the same space as at least two bedrooms in a house. It also, as noted above, is going to be poorly insulated, relative to the average house. This will require at least a 13,000 btu AC, and my guess is that he put in a 5,000 or 6,000 btu unit.
I, too, would not mess around with a window AC, but go straight to an roof mounted RV unit. They usually come in 13,500 btu or 15,000 btu sizes, and are mounted in the middle for efficiency sake. They will be only a little more expensive than a similar sized window unit, but will be far more satisfactory, not least of all because they are built to take the bouncing around the camper takes going down the road, unlike the window units.