There a a lot of things you "COULD do" to keep batteries from freezing, (and allow them to start your engine fast)....
Like drive south.
Or use light bulbs to keep them warm.
Or take them out and put them in a warm place.
Or rent a heated building, (or build one), and park the coach there.
However, the easiest
, and least expensive
, way is to simply keep your batteries charged, (which you should always do anyway,
regardless of the OS temperature).
BTW, I always try to maintain my batteries properly.
Could that be why my 4 house and 2 chassis batteries last 9 years?
'96 Safari, 136k miles