I found the PM.
Here is a simple diagram showing how 6v batteries in series are connected to yield 12 volts. Then for charging purposes, you connect the positive lead of the charger to the same positive terminal on the battery that is part of the RV circuit, then connect the negative charger lead to the same negative terminal on the battery that is the ground side of the RV circuit.
I did not say this before, but you need a 12 v charger - not a 6 volt charger. There are many good brands besides Deltran. Nocco is also popular. Our charger specifically says to not run a load from the battery while charging - that is why we isolate the battery with the switch. I think if the battery is under load, the constant voltage fluctuation will alter the smart chargers work and interrupt the charging cycle. It is also helpful to read your battery literature and see the recommended charging parameters.