Batteries and isolation

[Canadian_in_NJ]

I currently have two 6 volts in my rv. The batteries are connected to make 12 volts.
I have 170 watts of solar that do a fantastic job of keeping them topped up!

Because we go camping for extended times, we have a plug in 12volt/120 volt fridge/freezer. It has the option to run of 12 or 120 volt and have it connected via a cigarette lighter plug and alligator clips to a 12 volt deep cycle battery Which runs it most of the time.

When I start up the generator in the evening , I connect a charger to the battery that supplies power to my portable fridge freezer so it gets a couple hour charge. During this time I switch the portable fridge/freezer to a 120 volt Outlet in the rv.

What I am wanting to do is eliminate the need to connect an external charger to the 12 volt deep cycle batter and have it maintained by the solar which is already connected to my trailer.

Now I know I could just add this 12 volt single battery to my bank, but because it is a different brand and older than the two 6 volts and they may not get along and the weaker battery may draw the other down.

Is there a way I can connect my 12 volt deep cycle to the rv, but isolate it so it only receives a charge, and is only drawn down by my portable fridge connected with the alligator clips?

Is this even possible?

[Bob K]

Just spitballing here. What about a switch with two positions that allows you to switch between the two battery banks? In Position A, your two 6v batteries would be connected to the system. In Position B, your 12v deep cycle would be connected to the system. Something like this maybe: https://www.amazon.com/Blue-Sea-Syst.../dp/B0070YEEI8

You'd have a single ground to the system, but your positive would switch back and forth between your 12v and your 2x6v batteries. That would ensure that you don't have both "battery systems" connected to the system at the same time. Plus, that particular switch is a 4-position switch that allows you to connect both banks at the same time. Might give you some flexibility in the future. Otherwise, there's a 3 position switch in that same entry.

Just curious, is there a reason you don't have your 12v fridge pulling power from your 2x6v battery bank?