Maybe you should re-think the isolator/diode idea. In the 80's we had a TT and the battery went bad, and I did not know it. We became separated from our other party, so we stopped and waited for them. When we decided to go to the next stop, the van would not start. Went back to the trailer cord and parts were hot, so unplugged it. Found the charge line to the battery was hot, so disconnected it, and luckily got a jump from a county truck. If I had a isolator for the battery, it would not have killed the van battery.