OK, it took me a long time to finish this project (finish?) but I ordered one of those small and cheap
100ah Lithium batteries off Amazon (although I could have saved some money by going AliExpress) and I rewired the battery location to be in the same bay the 12v relay and the breaker panel lives under the banquette. There is a battery sized gap in the space between the AC inlet and the cords running in from a hole down to the generator and the battery bay. I ran the wires that I originally installed from the Xantrex Freedom XC to the battery compartment thru newly drilled holes, and moved the Xantrex LinkLite battery shunt and fuse to the new location:

This should be more power than the original dual 6v batts could store and it actually can take the full 80 amps of the 2000w Xantrex when charging. Got to 50% in 45 minutes from low battery cut off. I purchased the Renogy 60 amp DC-DC charger to protect my 150 amp alternator, but it only ever charges at about 30 amps so I guess I don't need it. Not sure how I was gonna wire it in with the Trik-L-Start anyway.
The battery is NOT BlueTooth capable or freeze protected so I'll have to turn off the 12v in the vehicle when in storage to not charge from solar below freezing, should stay at the level I charged it to before turning it off for the winter months, assuming I take it somewhere that it gets cold. I got a second
Deltran Battery Tender WiFi Monitor for this battery so they both show in the app. Sadly they company no longer makes them, had to get a refurbished one off eBay, so no upgrades to the software over time (like tracking charge level would be nice).
This frees up the battery bay for use as more storage, although it's not sealed like the other two basement bays are, it's like the generator bay, so I'll have to find a plastic box to keep things from flying out on the road.