As I said earlier, I've boondocked at -37 C (-34 f). If I were not in the Toronto area looking after a sick friend I would have gone on a pavement trip just to test things out at -40.
I know the Yamaha Generator (electric start) will fire up at -26 C(it was not happy but caught on the 4th try--with a three minute wait between attempts). I do have a magnetic block heater for it, which has never been used. I have not needed to heat the propane tank, but again I could do so with the magnetic heater.
I know that at -26 C (-15 f) it takes over 3 hours with all the energy from the generator and the furnace running constantly to warm up the "house" to temperatures where one doesn't need a winter coat inside the RV (about 10 C {50f}). The heat output is about 28,000 BTU's combined. That's why even an installation of LI inside the living quarters would NOT meet my needs, even if the batteries were equipped with warmers.
My own personal belief is that no battery bank should be in the living quarters, unless there is a sealed box vented to outdoors, but others may do as they wish. After all, it is a free country.
The battery bank, starter battery, and (now) generator battery are all maintained by a solar system.