Tom, I have a 1996 Rialta and have had similar problems. When I bought the Rialta the coach battery on the passenger side had cooked off ie boiled the water away from the battery acid in some of the cells. Ok I thought that was just due to lack of battery care. I replaced both coach batteries and went on a trip. The fridge didn't work on propane so I used the 12Volt coach batteries to run it when not plugged in to AC. At the end of my trip I drove many miles for 2 plus days. On the last day, I strongly smelled sulfur as well. At the time I thought it due to running the rear heater and suspected it was heating the Sewage tank. In retrospect, it was just the same passenger side battery cooking off like I had found it when I bought it.
I understand a bit more now. The coach batteries are recharged by 3 methods. First, the main vehicle engine charges them along with the main engine battery. Second the RV's Generator (Generac in my case) charges the coach batteries only. Finally if you are plugged in to AC landline power, it also charges the coach batteries. It uses a Magnetek Voltage converter that in my case is under my driver's side back rear bench seat.
I think what we have discovered is a design flaw in the overall battery charging system. It doesn't know when to quit charging. Somethings I have read say it always charges them at full rate. A high tech charger I understand would taper off down to a trickle charge. Best solution so far - treat the symptom. Add distilled water to coach batteries when they get low and monitor them so the plates don't get uncovered. The batteries need to be in the system when running the generator and I believe the AC landline. A disconnect for them may be needed when driving long distances. Use a voltmeter to check charging voltage when running engine, running generator, and when on AC. Best guess is 13 to 14 volts max charging.