Hi Bigfeets,
Have you done a power budget? How much capacity do you need and how much do you want?? Once you know how much power you actually need, you can decide between installing 2 or 3 batteries.
If you have your heart set on AGM's then I suggest that you take a look at the Group 31 Diehard Platinum's. These are made by EnerSys and are the same battery that is sold under the Odyssey brand. They are 13"L x 6.8"W x 9.5"tall... with your plan to remove the door, you just might be able to fit two of them in there.
IMHO, the Boondocker PM4B-75 & Honda 2K combo well likely never get your (~300+Ah) AGM to 100% SOC. Do you have plans to install any other charge source (solar? high output alternator?).
And finally, keep in mind that AGM batteries should never be installed in a sealed enclosure... despite the fact that they can be installed in a living space, they can vent hydrogen gas and this gas must not be allowed to accumulate. Plan for some ventilation.
Cheers
-Mark