Thanks guys for the opinions, and I like long posts, Hollywood, so thanks.
By accident yesterday, while dropping my front diff off at Slee yesterday, I got to see the 4 gallon version first hand. Very solid. I was impressed at the thickness of the plastic, and the rigidity of it overall.
It's not often that I would go on an expedition trip where I would NEED extra gas, but I like the idea of having it...for whatever reason.
My thought is to fab up a steel tray of sorts that I can place two stacked 4 gallon packs on. The tray would then crank up where the spare tire used to go, and then it bolts to the frame. It would protect the paX and keep them out of sight out of mind. It might not be super easy to get them down (unbolting the steel tray (skid plate)), but it's not often I would need to.
On my 100 series LC, I have determined I can fit a 3 gallon air tank, two 4 gallon rotos, and an M8000 rear winch all under the cargo area, yet not lower than the bottom of the frame. All protected by a skid plate, part of which is the tray that drops to hold the rotos.
Until I can build that. I thought the rotos are perfect for temporarily mounting to the roof. I'm running the Kokopelli trail in April and need to bring the extra gas for that trip. I figured if I buy one roto now, it sets me on the right path for this future plan.