"A simple gravity feed with an on/off switch is outlawed in many states and is very dangerous."
I do not believe this is a correct statement. Name one state where that is accurate for auxiliary DIESEL tanks.
I also do not understand your comment about nozzles or clearance issues. My tank sits up against the cab.
I have the 40 gallon RDS tank/toolbox combo with gravity feed. The tank weighs under 100 pounds and 40 gallons of fuel weighs about 300 pounds. It was not expensive. It works perfectly. I now carry about 80 gallons and can go over 900 miles if I have to. No, my bladder can't, but that's what interstate rest areas are for.