It's a good discussion of the pros and cons, and to listen to the opinions, suggestions, and practical experience of others.
In the brochure, the Axis offers an optional 2nd auxiliary battery, so I'm guessing there's already room for it somewhere. Depending on what they offer as the primary battery option, I would probably diy the battery bank expansion. I know I can do it cheaper myself, as I've already done it on my current motorhome, and a previous towable. Buying and installing 2 GR24 or GR27 deep cycle AGMs, isn't necessarily that difficult, except that the darn things can weigh anywhere from 50# up each, as Gary says. I'm not sure of the alternator/generator/converter output, so I have not figured out how long it would take to recharge 2 partially drained 100Ah batteries, but the ones I have now can usually recharge after use in a few hours of driving, using an older (ancient?) 30A converter charger. I would guess the Axis convert/charger is probably more like 45A-55A, but that's another question to be answered.
As for an inverter, you can pick up a decent 2000W pure sine wave inverter, probably for less than $1,000, or if you're not planning on running any really high tech electronics, you could go with a modified wave inverter for about half the price of pure sine wave. You can source them from lots of places like electronics, automotive, and even some RV shops. They all have sales once in a while.
I'm not completely certain, memory fails or I didn't see it in my whirlwind tour at the RV Show, but I believe the aux battery compartment is located on the passenger side, just aft or just forward of the door. If it's located aft of the door, you could probably run the heavy inverter cables from the aux batteries, up through the floor and into the galley area, that is also located on the starboard side. If not, it might require some additional planning and math to get the cable sizes correct for the distance traveled, or you might have to locate the inverter to the port side, but it can usually be done with satisfactory results, if you take your time, and read up on it before you start. Some auto-electric or RV shops may be able to do it for you for a price, if that's an easier option.
I'd add the extra battery and a higher capacity inverter to run our coffee maker quietly, or without having to run the generator, or when completely off the grid at a non-serviced campsite or other boondock location. My coffee maker is a Hamilton Beach with an insulated carafe, that draws around 800W for about 15 minutes of brew time for a full carafe, then shuts off. That would probably be the largest draw an inverter would see in our unit. But, an important one. Caffeine is an important basic food group.
As for the OCCC question, camperAl made the (common sense) point that the number of times you'd be driving around with every fluid tank full would be extremely rare, but it has to be considered as a possibility in the calculation. If you traveled with just the gas (330#), propane (40#), and fresh water tanks (350#) full, it would use 720# of the OCCC. That would leave 1500# or so, for the rest. So, I thought even with the 3 main tanks completely full, there's probably a reasonable amount of excess capacity available, for loading it with humanity and sundries as well.