My van came with a single 600Ah LiFePO4 battery under the rear jackknife couch/bed. On trips in cold weather, I don't do anything but use it, as the interior temperature keeps it well within its operating temperature range using propane heat with a 12VDC blower and controls. Unless snow covered, the meager 300W of roof solar seems to keep up, though I haven't pushed it very far since we're off and running the next morning and the second engine alternator recharges the battery while driving.
For winter storage, I drain it to 50% SOC, then wrap this blanket heater around it. The longest version (160W) coincidentally is exactly the right length.
I use this Inkbird thermostat to control it, and a slide dimmer for an incandescent light to reduce the heat output to where it mostly runs continuously. The temperature probe sits on top of the battery, in the middle, and I cover the whole mess with a thermal blanket.
The battery manufacturer recommends 60F and 50% SOC for long-term storage, so that's where I keep it. Charge controller and battery switch both off.