I don’t really want to put my batteries in the camper I was gonna put them in the shed we live here in Michigan sometimes in the winter it’s -3 to -4° But I wanna prepare for even colder just to be on the safe side I was trying to find a heating pad that run off 12 V
Depending on how much charging/discharging the batteries do,
Insulation is probably going to do the job rather than consuming power with a heating pad.
Batteries generate heat, particularly when charging...
The 'Trick' would be to keep cold from creeping in, and heat from escaping, so batteries pushed together would be thermal mass, and insulation all around would most likely do the job.
Heat escapes 'Up', so thick insulation on top, cold has to creep in from below, so if heat can't escape cold can't creep in.
I did a plastic 'Tub' the first year and figured out two things,
I didn't like standing on my head to service the batteries way down in a 'Hole' (tub),
And it made the terminal maintiance a pain in the ass.
The next year I flipped the tub.
'Lid' on bottom, insulation board under and around batteries, taped around the outside of the batteries,
(I used 2" styrofoam board wrapped/taped in plastic sheeting)
And glued insulation board into the 'Bottom' of the tub (now top/side cover) and about 3" around the sides so I could get direct access to the battery tops.
The tub 'Bottom' and side insulation sit on the insulation I had the batteries wrapped in and sealed up well.
I used a rubber bathtub mat on the battery tops to keep my insulation from getting eaten by the terminals
It got -20°F that winter and I didn't have issues.
Just an idea if you can use it.