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.
The 93 has brass ball valves on every coolant line in the system. The 14 has one shutoff valve for coolant to hvac system which only interrupts flow. Doesn't isolate anything which sucks when your on the road and have problems. Hose pinch off pliers or a pair of needle nose vice grips with a couple of deep sockets stuck on each jaw will work. The only thing aside from the ball valves on the 93 is the cable operated valve on the firewall that controls flow to cab heater core. No coolant control valve at bunk heater core. Must have cab heat slid over to warm or bunk heat doesn't work. Not sure how the 14 is set up.
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.
