Bryan, Thanks for the pictures. Nice job I'm sure it works well.
We also have a 1991 Scamp
that came with a window mounting air conditioner. We are considering mounting it in the same closet. It is a 5000 BTU unit that draws less than 5 amps.
Originally my concept was to cut out the floor and mount it standing up so it's hot coils would be hanging out below the trailer. The cooling coils would than be inside the closet and ducted to the front.
The concept would have allowed the hot portion and it's cooling air to be totally drawn and exhausted to the outside air. As well water drainage would not be an issue. During travel it would be protected by a snap on box.
It turned out that the floor of the closet is big enough but the area you can cut out is not by a few inches due to the width of the steel frame.
Now I'm thinking of a solution similar to yours where I virtually divide the box into three volumes. However I do not want to cut thru the side of the trailer.
The AC unit would stick out the front of the cabinet a few inches like yours.
The remainder of the unit would be blocked in construction foam to create a second volume from the front of the cabinets to the back side of the side vents of the air conditioner. The vents' air will be sourced from the cut out bottom of the trailer's closet and provided by a 12 volt muffin fan.
The third volume will be 'hot coil area". This volume will be from the rear of the side vents to the rear of the trailer. This volume will also have a muffin fan to push the hot exhaust out.
Volume two and three will be seperated below the trailer to prevent exhaust and input air mixture.
Water would be drained via a tube.