NEWARK and MOUSER sell the PB137 it is a 13.7V version of the 7815.

13.7V is the perfect voltage for Sealed Batteries, you NEVER EVER WANT TO OVER CHARGE THEM 14.1V is the absolute max.

GEL CELLS will never heal from overcharging, the bubbles form and push the mat away from the plate, the bubbles do not dissolve or move.

I like your idea, you could measure the amount of current flowing IN to the batteries if they are not receiving then you can by pass shunt off the excess.

On ebay many folks are sellling dc-dc converters, you can use these to stabilize the power from the panels