Wow, lots of useful info! Very glad I asked rather than just start blindly trying different things. First off, I totally agree the furnace is the biggest draw, if we use it. Frankly we don't use it that much, and we never use it at night - too loud. If it's warm, obviously don't need it. If it's cold we do use a cat heater at night. It's that "in-between" weather where it gets some use. Not cold enough to set up the Mr. Heater portable Buddy, but nice to have some warmth in the morning when getting up and in the evening when getting ready for bed. Realistically it probably isn't used over an hour a day, so maybe 10 Ah.
Light usage is pretty inconsequential. The water pump probably isn't too bad, but we do use it for showers and doing dishes, total water usage is no more than 15-20 gal/day. Not sure how many Ah are required to pump that amount of water. The fridge does not have electronic control and runs exclusively off of propane when dry camping.
Anyway, based on what I've read, I definitely don't want to parallel the batteries. Inverter/converter sounds wasteful because of losses on both ends. I really like the idea of using welding cables to just run the pup off of the truck's house battery. What I could do is start out with the on board batt until it gets down to 50%, then disconnect it and switch over to the truck batt connected with the welding cables. Also, maybe I need to revisit solar, even shady sites get some sun. Sounds like definitely no magic bullet, but instead a combination of conservation, more battery capacity, solar, and maybe still some truck charging (if I can figure out how to connect it properly).
Btw, we really don't stay longer than 5 nights. Assuming a max of 20Ah/day, I don't need more than 100 Ah total. I have 2 group 24's, so without buying anything else I have ~ 65-70 usable Ah right now without any charging. I'm not that far away from my goal...
