I'm confused? Did the rv repair place only call on the $210 hose or the $3400 AC unit initially? (assuming the total $3685 bill includes the hose too)
If so, it sounds like the rv repair shop was only "pre-authorized" on the hose repair and then "found out" the whole AC unit was garbage and tried to toss that into the bill as well??
I'd be questioning the rv repair shop at this point?
Is the AC unit a heat pump by chance? No way a roof top AC unit would cost that much.. That's about what I paid for my home heat pump about 10 years ago..
Good luck!
Mitch