Low volts at idle

Sounds like you have excessive voltage drops in the wiring. The battery alone should supply ~12 V to the cabin if it is OK and charged. The alternator is just to recharge the battery over the long-term. Step 1 is clean the battery terminals and insure the cables aren't corroded there. Step 2 is to find the voltage drop in the system, using a multimeter. Likely culprit is the bulkhead connector. The terminals in it can be removed and cleaned. The connectors at the ammeter on the back of the dash can get corroded, and get so hot as to melt things. All current from the battery to the cabin goes thru the ammeter.