From my recent experience losing most of my brakes and watching the repair, there is nothing particularly different about an RV system, especially from that era. Sounds like there may be some bad fluid in the system, or maybe sticking calipers? Just guessing. Oh, the master cylinder is usually hard to get to, especially to fill.
I expect you'll hear from some more knowledgeable people here, but welcome to the forum. Please post later with the results and any more questions you may have.
Mine lost a right rear caliper seal which took out all the fluid. The caliper overheated from the pad being worn down, and the backing plate was digging into the rotor. I have a tag axle, so I had a little bit of electric brake still available.
Ken, Judy, and the Angels--2005 Fleetwood Southwind--2008 Cargo Trailer--2003 EZGO Golf Cart