Fix your dash AC so it works and replace the roof AC with whatever AC unit fits your budget. I wouldn't even consider driving one of these motorhomes without the dash AC working.
I've been working on Peanut's dash AC and it's a pretty straight forward system to work on and for replacing parts. You can see Peanut's thread and also there are a log of comments from Gregg Wallace who has also repaired his 77 dash AC.
The roof AC unit is a pretty straight forward swap for a new unit.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)