If you have two group 27's you have less than 200 amp hours total
Replacing them with a pair of GC2's (A bit taller but smaller at the base) and some shims (Boards) will get you 220-230 amp hours of DEEP CYCLE battery, this means you can safely use half, a better chance of recovering if you go over that and possibly a longer life,, Get 'em from Sam's. and they (if you are a member) are about the same price. This is a far superior battery to what you have now.
As for the chassis battery,,, Replace it, I leave the choice up to you.