My Coachman put the dash A/C in front of the fire wall inside the engine compartment and tapped it off the blower box (It is a Ford F53 chassis.)
But a more logical location would be to put the blower box and all ductwork INSIDE the coach (on the firewall) and under the dash on the passengers side...
Unfortunately, it that's the case, you'll have to pull the passengers seat so you can get access to the space...
You'll be looking for a box that looks like this...