I do have a diesel with dual batteries. I have also forgotten to take my fridge off DC and then ran my battery flat. I’m reluctant to gang all three batteries together in case I do that again. Also, my three batteries are not the same type. Why would I have a diesel starting battery in the camper? I still use DC for the fridge when on the road to save propane, since I have only one 20# LPG tank.
You mentioned your TC being in it’s shed and not having the sun to keep your batts charged. My TC is always outside but on inspection I noticed that my solar line is disconnected if I pull the batt disconnect switch. I wrote Lance and was told that it was “by design”. I don’t get it. I no longer pull the batt disconnect.
I’ve taken your suggestion and just ordered two Lifeline 6 volt AGM’s. (Now I have to hide the credit card statement from my wife.) With AGM’s eliminating the venting requirement, there’s plenty of room in the 825’s batt box for the two taller 6 volts.
You wrote that your TC is in the shed “for months and months”. There’s the problem. Get traveling 🙂