This is a tough call. My first rule of Desert travel is to top off the tank at any available gas station and keep the 2 x 5 gal tanks in reserve. During our trans NV trip we filled up everywhere we could and ended up carrying 20 gals of fuel the whole trip. It did come in handy on the way back from Reno to Phoenix though, we drove almost straight through.
On the cruiser it was easy because they were mounted outside, the pickup was the same way, they were mounted in the bed.
With the 4-runner though it is a little different, I carry a 5gal plastic inside. I hate the idea of it though. In reality I don't do long trips in the runner....yet.
I am almost done with my little trailer, so on long trips I will keep extra gas in there. I have toyed with the idea of the 11 gal Auxillary tank though, but that comes with it's downsides, like relocating the spare.
A 400+ mile cruising range is optimal in the lower 48, barring an extreme emergency of forgetting to top off somewhere, you would rarely need more than that.