I realized I may have misunderstood. By “main” battery you mean the chassis battery, right? Nothing in the coach should cause it to discharge. Either the isolator has failed- as you said- or something on the chassis is causing it to discharge.
Mine had a problem with the chassis battery, too, and I eventually discovered a short circuit in the center overhead console where the dome lights are. It’s part of the chassis wiring, but a wire was damaged during the camper upfit. I had forgotten all about that stage of the electrical adventures I had after I got the van.