The dash A/C on our (then new) '18 Miramar was completely empty when we received ours. Of course, we took delivery in February and this was our first motorized unit, so the dash A/C was the last thing on our minds. We did have the coach A/Cs on our PDI checklist, though. When we brought it up with our our Dealer, they wouldn't touch it, saying it was a chassis issue.
We have a friend with a similarly sized Forest River Bounder also on the F53 chassis, which they also picked up new a few years before us, had the exact same problem. He ultimately filled his with one of those automotive A/C recharge kits and it's been fine ever since. It seems that neither the manufacturers nor the dealers care about the dash A/C. Taking it to a qualified repair place (e.g., a Ford Truck dealer) to have it evacuated, tested and filled is the correct way to get it working as it could have contaminants in the line which could shorten the A/C system's life.
I feel you, though. Much like a lot of others, we had lots of relatively minor, annoying things wrong with ours as well. I've fixed everything myself because the rate I discovered the problems was just far enough apart that it made more sense (time and effort) for me to fix it than spend the four or eight hours drive time (two hours down, two back, or double that if I dropped it off) to get the dealer to fix them. I tried the dealer on one of the problems, but suffice it to say they weren't able to fix it when I told them where it was. When I finally fixed it, lo and behold, the problem was exactly what I told them.