maximizing battery capacity

14.5" wide by 20" tall and 9" deep

Hummmmm. Thats kind of an oddball size for a battery compartment. Just a little too small in each dimension to be efficient. My 6 volt deep cycle batteries (232ah) are +-11" tall (plus you need space to service them) so stacking them on your rig is not an option. With a little research, if you want to convert to 6 volt wet cell deep cycle batteries, you could probably find batteries with somewhat less than 232ah capacity. If you could find batteries that were about 8" tall at the fill caps you could run 6 volt deep cycle batteries in series and stack them. It would be a tight fit. Another option would be to find a specialized battery for something like a fork lift, a pallet jack, or some other warehouse type equipment that is much taller than a standard deep cycle battery. Those could be pricey though.

Another option (which I wouldn't recommend) is to use two 12 volt automotive batteries wired in parallel. They would fit, stacked, but only the highest quality batteries would give you minimum life. Marine deep cycle batteries, which are generally not true deep cycle batteries are another option.

AGM batteries can mount in any direction so that might help you.

Any upgrade suggestions from the camper unit manufacturer?

Chum lee

Have you ever read The 12 Volt Side of Life? Maybe it will have information you can use.

If you do find the space for two 6V batteries, you will want to connect them in series, not parallel, if you want 12V from them.

Trojan T-1275 (150 AH)

Length 13" x width 7.13" x height 11.13"

Absorbed glass mat will render more kWh than a flooded L/A battery. They can also tolerate unintentional over-discharging better than a FLA. But brand is important. I find it difficult to fault Concorde's Lifeline. This brand has thicker plates than even a Trojan T105 Golf Car battery.