I have not installed extra batteries yet in my Axis.
But I have given it some thought. I would also add to this compartment ( next to the existing batteries and Under the sink area). But I would NOT "replace" the existing compartment but rather reinforce it some how.
I would want mine to look OEM and be waterproof (except maybe vent or two near the top?).
I was thinking of in internal angle iron that is predrilled (around 1 1/2 inch wide) that can be cut and bolted together at home and then either bolt to the floor or bolted under the rotocast compartment and up to some other metal structure that you can drill into and bolt to. The metal next to the step is very thick so that could support one side.
You might also consider AGM's since you would NOT have to vented them plus it might be easier to make the connections to the batteries if they are laying on their sides. Not to mention this compartment is very short so you may not have any other option but on the side?
Of coarse this would mean all 4 of your batteries would need to be AGM's of the same size and close to the same age (unless you split your system - have the converter charge the OEM battery and solar charge the new ones? But then how to you hook them up to use the juice?).
Or you might find room under the RV to bolt two more batteries behind the steps? But they would be very hard to hook up (no room) and check on.
Let us know what you figure out. But I like the idea of doubling my amp hours some day (esp after I add solar).
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor