Is this even a thing? My RV battery froze last winter and had to be replaced after nine years of good service. I disconnect the negative terminal when I put it to bed every fall, but it sits in the engine compartment for six months or so and it does occasionally get below freezing here.
Short of removing the heavy bastard and putting it back in twice a year (getting too old for that), is there any way to keep it warm? I don’t think a trickle charger will serve that purpose, will it?
Yes, they are definitely a thing. You can get them at just about any auto parts store or from Ama***. Unfortunately, they need 120 VAC (or other, if you are in a different country) to work. But a warm battery is great for starting a cold car. I had quick connects on my battery and toted it up to my apartment every cold night. Pop in the car the next morning and it would start instantly.
Ask most Canadians, it is indeed a thing. I would recommend a timer as well otherwise the thing stays on. Just remember to take it off come Spring or you’ll be putting 4 cups of water back into your battery like I did with my Jeep…Apparently boiling off your electrolyte is like, bad, or something… 
Battery Blanket.
That’s perfect. I’ll only plug it in when we’re dipping below freezing, which normally only lasts a few days at a time. In Alaska, we all just had block heaters. I do remember when I was in college in Fairbanks that the temps would get so cold that oil would freeze and gas would gel. I recall one student who lit a small bonfire under his oil pan, trying to get things moving, but at -60F, it was pretty hopeless.
Step back a second. It only gets below freezing a few times a year? Your 9 year old lead acid batteries did not fail because of the cold. The cold just hastened the end. Personally in your situation, I’d invest in a solar powered trickle charger to keep the charge topped off during storage.
As far as I know, a fully charged battery will not freeze, so because of that, a battery maintainer would also help.
My '62 Beetle had a little 6-volt battery under the back seat. It was easy to carry it in the house on cold Quebec nights. Things were simpler in those days. But I can’t really say if it had more cranking power at room temperature, than out in the street. Still had to drag the crankshaft through tarry oil, and heating the pan would probably be more effective.
However, since '62 Beetle engines didn’t have oil pans…
That wouldn’t have stopped me from trying.