MalachitePropane or Mapp gas with a burner and solder it in. Then heat shrink.
Overcharging and outgassing without good ventilation, or batteries on the verge of going bad do the most damage reacting chemically with copper and corroding it green. All that spare hydrogen and a pinch of CO2 from overcharging forms malachite green all over the exposed copper. Get rid of the hydrogen, the overcharging, or get some proper ventilation, and the problem goes away.
That solar system of yours is doing a bang up job of topping off your batteries. Perhaps too topped off?
Don't AGM batteries vent off a lot less to nothing, therefore preventing the corrosion on the terminals? Just another option, they aren't supposed to out gas?
If you have a hydraulic crimper in the RV down in Quartzsite, with all the rich old RV'ers down there, you could have the equipment pay for itself in one or 2 side jobs for others?