[AzB]

I am curious about the the self heating reducing battery life? Is that because the temp of battery will not be uniform?
Or is there something in general with regard to the LiFPO4 chemistry at lower temps

Its because the batteries use their own power for heating. Thereby running the batteries down.

 

[AzB]

Took a bunch of work but I have 200 Ah of AGM beneath the seat. The biggest worry was about having the posts face each other so there are very hard limits to prevent battery movement.
See Post #28 here: Checkswrecks thread for a minimal build stealth camper -...

I have a 120ah agm under the passenger seat. I could fit another, but it would require some minor trimming of the seat base, and I'd have to create a retention system. Right now, I use a ratchet strap for the single battery, but there wouldn't be room for the strap with two. It would require a metal bracket or two.

So yeah, some work would be involved for two.