No or low air flow, no heat but ac works

Ok so many questions I hope you can help with.

I'm wondering what makes you think that? Where is it located and is there anyway to test it before replacing it?

Any help is appreciated!

 

Ok so update..the ac has pretty much started working again but still no heat in the cab whatsoever. The bunk gets hot just fine.
We have replaced the heater core already. Does the evaporator coil have anything to do with the heat? I read that simply checking a few hoses for hot or coldness can help determine what's the problem. Winter is coming and having only bunk heat just won't work.

 

We replaced the heater core ourselves and drained all the antifreeze at the same time.
Honestly I wasn't aware of any valves, that probably would have made things much easy on that repair. It won't hurt to check so where are these valves your talking about?