Looked on line and every okanagan truck camper I see has the a/c mounted behind the cabover. While your truck camper may be prewired for a/c, are you sure it goes to the front opening (pulled the inside trim and seen the wires)? Just behind the cab-over, there is usually a blank cover outlet which is the location for splicing in the a/c which makes it very easy to install. This area will also be re-enforced to handle the extra weight (usually double joist).
May want to take some time and see if you can find someone with the same model as yours to get some dimensions.
A word of warning, make sure you don't oversize your unit. we have a 13,500 in out 8' camper and since it cools so quick, it never really has a chance to pull the moisture out of the air and everything gets a damp feel. If I was planning on keeping the unit a long time, I would go with a 8,000btu or such to make it run long enough to pull the moisture out of the air.
