I'm cleaning out the garage and found my PowerStream DC to DC Battery Charger PST-BC1212-15 gathering dust. Maybe you'd like to experiment with it.
I used the charger to charge the house battery in my Four Wheel popup camper from my truck charging system. Now that we've moved up to an Escape 5.0, the charger just sits on the shelf and needs to find a new home.
The BC-1212-15 inputs 10-15 volts DC and outputs 7.5 amps. It has a microprocessor controlled 4 stage adaptive charging algorithm suitable for gel, AGM, VRLA, and flooded lead acid batteries.
Because I need to list a price here on the forum, I'l ask $50. New, the cost is $195 from PowerStream. But make me an offer. Shipping is extra. The package weighs 4.5 pounds and is 11"x8"x4" so you can figure out shipping from Zip Code 98225.