I decided to go from one 70Ah battery to a pair of 115Ah batteries, thus tripling our power storage.  Many people recommend a pair of 6 volt batteries in series, but I decided to opt for a pair of Walmart 12V batteries in parallel.  They’re available everywhere, and come with a two-year warranty, and they’re only $100ish each.

I first removed the support tray for the LP tanks:

Next, I used the angle grinder to remove the front edge of the forward L-channel so the battery boxes could be installed lengthwise.  Here’s a mock up. 

With the LP tray all the way forward, everything just fits:

To finalize the install, I installed aluminum L-channel to support the batteries:

It’s a tight fit, but it works!

With the LP tanks so far forward, the cover actually encloses the front jack, so I had to cut an access hold to allow to cranking:

Lots of wiring to come!