It sounds like you're on a parallel path to what I have in my
motorhome. It had 2 tired deep cycle batteries which I replaced and
added a third. If I remember right I have about 1kwh capacity to 50%.
I bought a 1000W true sine inverter on sale for $150. Now I have more
battery capacity for running the furnace heavily overnight when dry
camping in cold temperatures. And I don't have to use the generator
nearly so often. Even the inverter microwave is OK at half power.

When my two twelve volt batteries died, I got two mondo six volts and wired
them in series. They lasted a lot longer than the two twelves, and I would
suggest it to anyone who is at a stage of buying two new twelve volt
batteries for their MH or TT. I can't explain the math, but it does give
you more amp hours. Costs about the same. Sure made a big difference in
our coach on how long the batteries would last between charges, and
intensity of lights.

Steve