If the compressor senses low charge it will not run, and if it does not run it won't let you charge, kind of a catch 22.
In my front drivers side run bay there is one 25 amp fuse in the larger panel and then in the #1 Monaco box, smaller mounted below, there is a AC relay and an AC circuit breaker.
I had the problem when I tried to charge my AC. Have the two gauge setup and large bottle of refrigerant. I just had to keep adding a little at a time until the compressor started. I know there is a pressure switch on my lines that controls this and you might be able to connect the two wires to fool the system into running to be able to charge it.
There should be a description in your manual that has a table showing the pressure you want to charge to, I put my high side at ~175 psi.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi