If all you are worried about is keeping the batteries happy install some sort of battery tender/minder. Pull in, turn off the battery disconnect(s), hook up the tender/minder and forget about it until your next trip. Every thing I own that sits for any time period has a BT JR (.75A) hooked up to it. Multi-stage converters are great when you are on the road but when in storage you want the lowest possible charge/maintenance rate possible. All of the minders/tenders, the lowest amperage possible, have the least negative long term effect on your batteries.
Randy