Originally Posted by BFlinn181
Most likely your dampers and diverters in the HVAC plenum are vacuum powered. A leaking or unhooked hose, or a bad vacuum 'motor' is likely the issue. If vacuum is not hooked up at all, usual practice is for all air to be diverted to defrost, so it's working a little.
On the heater side of things, and I'm talking Engine/Chassis heat thru the front dash here, sometimes the hoses go bad or as mention 'valves' don't open up and allow hot fluid to circulate to the area. I've seen when inspecting used coaches, where hoses have been 'bypassed' closing off full loops of hoses to the front of the coach.
On AC, I'm not up on these enough to know why the passenger side would work while the driver side was not. I loose or crushed hoses could be involved..
Best of luck on solving this, and I'll follow along to learn with you,