a few years back we kept our rig in heated storage that provided power 2 hours each day. maybe my solution could help you. I turned off the use switch and ran extension cords to power stand alone separate trickle chargers to each battery. you could do the same with an appropriately sized timer(s) to keep the batteries charged.
what I do now in my unheated pole barn is shut everything down an charge for a few days every 3 weeks over the winter months. it's ok for batteries to draw down on there own as long as you charge them up again.
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