^ Yeah, so much for real world advice and NOT diving into the weeds....
I think the simple answer is move the batteries inside the camper somewhere. Storage and discharge below freezing aren't so much the issue as trying to charge a cold battery. Presuming you arent letting the camper cold soak down well below freezing and at the same time expect to be charging the batteries, that's a good compromise since you're not really going down into extreme cold temps.
Remember it's the battery "core" temp that matters. I use a LFP on my snowbike. It is certainly in the "cold" all day. Older original battery needed much more "warm up" before it would discharge at a fast enough rate. Couple bumps of the starter to load the battery momentarily and then off to the races.
If you are to the point where insulated boxes with pad heaters are needed, just scratch LFP as being practical. If you can do the above suggestion to keep them warmer without extra effort, then IMO it will work fine.
