I agree with the three 12s in this case. Last time I had three 12s I just treated them as a pair, with loads on the outer ends. I think this gives the middle batt an easier ride?
So with one pair of 6s perhaps getting shaky (some black stuff in hydrometer if do four sucks a cell, but clear if only do one suck per cell---which is what I do now, so they look good--out of sight, out of mind) although seemingly at full capacity, and one pair in excellent shape,
I was thinking of putting the good pair of 6s in the middle with loads on that, with a pair of batts on either side as "downstream" so those two pairs on the outer ends get an easier ride and the good pair of 6s in the middle takes the weight.
I can't add one 12 to each end with the four 6s in the middle due to compartment sizes. Has to be either a pair of 6s or a pair of 12s in the middle of the three pairs.