I have something close to 1solmofo's setup except I didn't get a custom made tank, I bought a RDS 45 gallon transfer tank from Northern Tool. It's tall and "thin" so it would fit in front of my (already) installed toolbox. I suppose I could've sold the toolbox and bought a different configuration but this set up works for me. We have a long box Dodge so the tool box and fuel tank don't take up that much space and so far hasn't been any trouble.
I LOVE the extra tank! I now have about 70, 75 gallon fuel capacity which gives me almost 700 miles of range. When we have a long day I can easily get to the campground, drop the trailer and go to ANY gas station I feel like. Easy in, easy out. Case in point, while in Oregon last fall we were staying near Portland, I found (on Gasbuddy) a Safeway that had fuel for 25 cents a gallon less than the truck stops along the freeway. No way in he** would I have gotten into that place with the trailer on! Needles to say I topped off both tanks! I didn't get the extra tank so much for fuel savings but for range and convenience, the savings is a nice added plus.
Dan