Pretty much nothing available, assuming a Suburban furnace which is what I assume you're asking about.
The furnace draw, IIRC, is under 4 amps. This should not kill 200 AHs of fully charged lithium batteries unless you are using a lot of power elsewhere. 12 volt fridge? Incandescent lighting? Starlink? CPAP?
Do you have a shunt based battery monitor? pretty much required with a lithium setup and highly desired for lead acid batteries.
Some folks use a catalytic heater, with proper precautions. I'm not a fan.
Tick tock, baby(Ironbuttal)
2000 Roadtrek Chevy 200 Versatile(sold)
'98 Safari Trek 2480
Just for fun:'15 Kawasaki Versys650LT
Perfection is a fantasy, though improvement is possible(Wifey).