I just use a self-service can of refrigerant with a gauge to fill mine. The gauge displays the correct fill. It works well for our MH, my truck, DW's Jeep. The most important item is DO NOT OVERFILL! R134 works poorly when too low and when filled too high.
The A/C system for a MH, auto, truck, work the same way.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"