Disconnect the batteries and take them to a store that can put a load on them. You will find out if a battery requires replacement.
1. I suspect a battery is partially bad, able to accept a charge but will not hold it under load. Sometimes you need to add acid to the battery, most times you need to replace it.
2. Taking the batteries out will tell you if your cable connections are ok. Sometimes there is corrosion under the connection that you do not see, but interferes with the batteries function.
3. Last, and not uncommon, you have something in the coach that is drawing more than the battery charger can replace. You mentioned a disconnect switch, which is good. If your batteries are ok, your converter seems ok, then the next step is mega, finding the fault in the coach.
07 TM 40D;ISL 450 Cummins(upgraded). Very happy w/coach
Gnarly toad to scoot around.
Geocaching, hiking, fishing, National Parks, Civil War sites.