Forced air anything is out of the question once you leave the electricity supply. Forced air blower fan motor takes average of 5 to 10 amps. Hot water circulating pumps take 0.5 amp.
Lead acid battery's freeze all the time. They live under the hood of every car and truck on the road.
What they don't want is to be froze after being discharged. This kills them almost every time.
Keeping them above 0 C will give you more usage or run time. Insulating the battery's on the sides and bottom, but not sealed at the top is key. Styrofoam is best as it is not affected by the corrosive gas.
I would use the thermal mass of the water tanks to keep the battery's the same temp. Snuggling them up between the fresh water tank and the grey water tank would work best. All you need for a vent is a tupperware container silicone upside down over the vents on the top of each battery, with a pex line running to the outside.
Better would be to use AGM battery's. No venting needed.
So now that I understand your needs (Boondocking) This is the system I would install.
Propane hot water heater, Hot water heating lines, circulating pump, and two 8D battery's or equivalent in AGM. This system could run without human control for 12 to 24 hours, heating your whole bus including engine for start up. Simply add more battery's to add more run time.
This could be set up for around $1000.
Propane Hot water heater $400
Circulating pump $200
2X 8D Battery's $400
Pex by the foot $0.21
I have this system set up in a 750 square foot cabin. It works flawlessly.
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