Why not parallel batteries? That is how you get more AH.
We did that for a while where we had two 6s in the trailer's battery compartment, filling it, so we just sat two 12s (27DCs) on the ground (we carried those in the truck bed while travelling) and paralleled them to the 6s with the set of jumper cables we carry all the time anyway. Worked great.
We did need the two store-bought #4 "starter cable" wires to parallel the two 12s that sat on the ground. The jumper cables (10ft of #8 I think it was) joined the two pairs of batts to make a bank of four.
That was before we got the big inverter and had to tidy things up by finding a place in the trailer for the two extra batts and some fatter wire to operate with at the higher amps the big inverter draws using the microwave etc.
