You might be able to get a VA unit in there, but I think it'll be tight. As you have no doubt seen, they have dimensioned drawings of their various offerings on their website.
I think one of the challenges,will be to get the ducting through the firewall, especially if
You keep the original heater. The central rectangular port is tight, but maybe you can do
A version of what you have shown on the UHN, and tap into the existing plastic duct, on the right hand end. I think you might have to add a controllable flap to keep the A/C from
Blowing out the cowl intake. One good thing is that the VA controls are 'fly by wire ', so you
Would not have to rig up cables and sliders like the old days.
Someone mentioned the Unit that mounts to the rear cab wall- that is worth checking into,
And would be a simpler rig. The fellow that built the nice camper on here (4Xexplorer I think) has a unit in that position. There was also a U1750 armored SWAT truck that sold in the US with a similar mount. Red dot and Greentop both offered these I think.
If you still have the double military bench seat on the passenger side, I think you might be able to get one of the VA units under it, running the ducting up the back wall.Keeping the VA unit inside the cab somewhere will eliminate the need to put it in an insulated box, which
I think you might wind up with of you put it into the engine bay.
Another possible solution is to look at the units for construction equipment - Redtop makes
Some of them. They are similar to a rooftop unit, but they mount to the side of the cab on the outside, and are built to operate in dusty conditions, blowing cool filtered air into the cab. I looked into them, with a thought to having it feed either into the cab or the camper...
Although having the engine running when parked, just to run the A/C didn't make much sense. My box has a void up front that would take the unit, but I'm not sure where you could mount it with the Ambulance box.