One of the first upfitting projects we did on Blue was to add three AGM house batteries. I had read on more than one forum that it might be possible to install one or two of the batteries under the front passenger seat. I couldn't find any threads indicating that anyone had actually done it, nor did I find any information suggesting that it couldn't, or shouldn't be done. So I removed the front passenger seat and took some measurements.
Sure enough, it looked like there was adequate room to mount one 105ah Group 31 battery, and I decided to give it a try. While there were a few complications and glitches, the end result isn't bad.
I did a video documenting the process which can be viewed here.