1975 GMC eleganza ll just purchased, certified. Dash AC not working. Best steps to address this?

FYI, I'm near Thamesford ON

Your best GMC motorhome Mechanic in S. Ontario is Borrmann's Garage in Bluevale (near Wingham).

The dash HVAC system in the GMC motorhome was never great. The air makes about four 90 degree turns before it even makes it to the dash duct-work.

There are 2 documents in the resources section. Read them first to understand the issues and the changes that GM made over the years.

 
Highly recommend you ditch the dash a/c, route those two lines to a standalone auxiliary A/C , under dash unit and mount it in front of the engine hatch.

I have the dash air working about as good as it can in my 76 and it's still not great.