You can get value from the spare 12 by paralleling it to your "big 12" 4-sixes temporarily using jumper cables. That will give you some extra AHs when needed for some reason. I have done this many times.
Normally charge the single 12v separately from the four 6s, but if this is impossible, and you are charging the 6s, leave the jumper cables on and the 12v will get some charge too.
I used to carry the extra 12v in the back of the truck and charged it with an extension adapter I made up for the 7-pin so it got charged some (not much really, but every bit helps) while we drove around sight-seeing or going to town.
With that much solar, if you get to near full on the 6s in early afternoon, you can swap the solar over to the 12v for the rest of the day using jumper cables.
It all works, as long as you don't mind managing and keeping track of it all so it doesn't cut into the camping fun any.
