When you installing an air conditioner in a closet, the front of the air conditioner functions just like it does in a window installation. The back of the air conditioner needs to have structure that lets it function like it was outside.
Their are two air flow paths in an air conditioner, a cooling air flow and exhaust air flow.
Cooling air Flow.
Normally air enters thru the front of the air conditioner on the room side goes thru the cooling coils and flows back into the room thru the top front of the AC unit.
This portion of the Air Conditioner is the part that is normally in the room.
Exhaust air flow.
The exhaust air normally comes from the outside the trailer and enters thru the side vents of the air conditioner and exhausts thru the back of the air conditioner.
In a closet installation it is necessary to have a cool source of air for the air conditioner's side vents. The side vent's air must be isolated from the trailer's cool air and the exhaust air.
Isolating it from the trailer's cool air consists of two steps: 1. sealing the front edges of the air conditioner, 2. a cover on the top of the air conditioner. This should prevent the air conditioner's exhaust fan from drawing room air into side vents of the exhaust system.
The next step is to provide cooling air to the side vents. I cut a hole thru the floor of the closet for a cool air supply for the exhaust vents.
It is also necessary to seal the side vents from the exhaust air of the air conditioner. If this is not done the side vents will draw cooling air from the already hot exhaust air.
To prevent the AC from drawing cooling air from the exhaust I place a piece of foam between each of the side walls of the closet (sometimes one is needed on top and bottom depending on the brand of AC) and the exhaust area. All these insulators/isolators are located just after the side vents (some AC units also have top cooling vents as well).
I also have a second hole in the floor for the exhaust air to exit the trailer. In my case I mount two small muffin/computer fans to push the exhaust down and out.
To prevent the hot exhausted air from routing itself to the cooling vents, I put a divider between the exhaust input (cool air) and exhaust output (hot air) under the trailer.
Hope this helps. If you need more information please ask. I think there are some pictures under Modifications/Preparing a 1991 Scamp