Sorry didn't see the question directed at me! The aux (transfer tank) that we have is made by RDS MFG and sold by 3Gtool boxes, $399. 3G Toolboxes
We have 70307 and is strapped in with 2 15K tie down straps for easy in and out. You have to get a diesal install kit for it, depending on trk. Basically what it is a valve with a floating stopper in the aux tank feed line. Theres one 1/2" fuel line that comes up thru bed of truck, 1 clamp. You have to cut a section out of the rubber fill line and install the valve.
Way it works is it gravity feeds into main tank keeping it full, the float shuts aux tank off when main tank is full. When you see your tank gage start to go down you got 26 gallons (Ford) left till its panic time! When we stop for couple days I just make sure to top off the aux tank and we always have 700 +- till empty. This is our 3rd year and I have not, knock on wood, had to get fuel with the 5er attached! Let me know if more info need I mite pics somewhere!