Bunk A/C- old vs newer.

Close or pinch off the coolant lines to the sleeper and see if if blows colder, if so the water valve in the bunk has most likely failed, have chaned many of these over the years.
There is no air filter on the HVAC system in the bunk OR the cab of a w900 or t800. only the add on outside filter pictured above.

 

Since you have to have slider in front moved to warm to get bunk heat, im going to guess that coolant lines for bunk heat is tied in a continuous loop with your cab, should be easy but annoying to change that over, but i kinda like it since you could set it so you dont have full blast heat in the back. Ill be setting up my brothers truck like that i think.