For a maintainer, I use the Pulsetech system. I got a decent deal on one that came with a power distribution box (great fuse panel for about 18 circuits of both 12V and 24V), a battery state-of-charge indicator, and a large solar panel that seems to do well with keep batteries topped off and desulphated. They have a simpler system that also works well, which is simply a small panel and a small pulse controller that keeps the batteries topped off (I also have one of these in a box...)
I do like your idea, especially if the plan is to use a much larger panel array to actual recharge batteries and not just maintain them. It still would be rather slow and probably not worth the cost of building it...but it undoubtedly would be a cool project and might fill a niche that you have that isn't commonplace. If you go that route, follow the earlier advice and install a proper charge controller than can regulate voltage of the solar panels and will not damage your vehicle's electrical system. Then when you're not just recharging, you can pull the plug on your panel assembly easily and use that slave port for anything you need.