Splitting up the batteries and charging

The dimming/slowing may just be not the best connections between the battery and the rest of the RV. Obviously the battery voltage will depend on the load to some extent, but a somewhat high resistance connection will exacerbate that.

If you have twice the total battery capacity and the furnace is using 60% of your capacity currently over the course of a night, it would only use about 30% after the upgrade and your lights would still be decently bright.

I don't mean to suggest that there's any inherent reason why it's impossible or unworkable to have a separate battery for the furnace, it's just I don't see any benefit. (Intriguingly, my house--and several others of the same general vintage--is wired much like what you're proposing. I have two independent 100A electric panels side-by-side in the garage, connected to the electric meter, originally one for electric heat and one for other things. Over time the separation of loads has become a bit mixed, with some non-heat loads now on the heat panel, but that was the original setup. I'd slightly prefer a single 200A panel given the choice.)

Huumm. Not that it's probably a lot more wire but more wire and more connections equal more resistance in theory. Drawing one battery down with x amount of amps vs spreading it over twice the amount of battery may reduce the Peukert effect. May be splitting hairs. One blue seas rotary switch can separate batteries 1/2/both positions.

Another 2x 6v batteries should cure those issues. A battery drain protector between the bank and the furnace will stop it from draining too much from the bank.

Have you switched your lights to LED yet, preferably the kind with built in regulators?

Gordon Three and Drew,
I've noticed when the heater is running on a cold evening, then we have a couple lights on and turn on the water, the lights dim and the heater fan slows. This would leave the heater alone when we mess with other 12v draws. Also, on a real cold night, say 20-30 degrees and the heater cycles through out the night and pulls the battery down to say 40%, I will wake up to nice bright lights and easy running pump. Really that's the whole idea.

X2, Drew. Much more simple.

Why have separate battery banks? Why not just add a couple of additional 6V batteries to your main bank? Having a separate bank just seems to me to be increasing the complexity and reducing the flexibility of the system...you can't use the primary bank to power the furnace should the secondary bank be run down (at least without some additional switching), you can't use the additional capacity for other things in mild weather, etc.

That aside, charging both banks together if they're at quite different states of charge can't be ideal, though I don't think it would cause any great problems either. As shown, you have the possibility to accidentally have the solar controller connected to no battery; many (though not all) solar controllers will be damaged if this is done, so check the documentation for yours carefully. It would be possible to eliminate this problem with (the right choice of a) single switch to choose between battery banks for charging.

You would need some fuses that aren't shown in your diagram. In particular, if both switches are closed and one battery bank is rather discharged compared to the other, a quite high current could flow between the two banks depending on the overall impedance of the connecting wire.

How is this an advantage to just running four 6v in 2s2p configuration?

Is the idea for an isolated battery, such that when it runs dead, you still have reserve power for what, the coffee maker?

Maybe a Blue Sea battery protector connected between the furnace and the 2s2p bank, set for 12.4v ... battery protector cuts off the furnace when your bank is low, but before it's drained completely.

No ideas on the solar, I get my boondock electricity from used dinosaurs.