HI ROY

I have a 75gal. tank. That I have a pump off a 2000 diesel chevy. I made a section of pipe that tapes in to the fill hose. It has two hose barbes in it the closest to the main tank is the return to aux. And the upper one is the fead from the aux. this way if you foget to shut it of when main reads full it returns to aux. I also put A vacume brack befor the pump to stop a sifon from forming. The vacume brack I got at West Marine. Also wired the switch to ign. hot. Have forgotin it more than once with no over flows. Good luck Mike