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I have an '02 Sierra 2500, with the 6.0L vortec. I tow a fifth wheel fairly often, and stopping every 200 miles to get fuel is a bit bothersome, since you never really know the quality of the fuel up in the mountains. I found a complete tank, fuel pump, and filler neck out of a different truck that bolts between the rear frame rails (will go in place of the spare, I have a dually rear end swapped underneath my truck and I hate the spare tire location anyways, so might as well use the space). Bolting it up is no issue, but I'm not 100% sure on routing fuel lines.
Electrical will just be a relay and a switch on the dash switching which pump to turn on, using the factory fuel pump wiring. Same with the sending unit if the second tank will work right. Otherwise it might just be a separate dash near the switch. There are the usual 4 fuel lines- vent (up to the frame rail, easy enough), filler neck (to the bedside in the rear, where it'll have a universal fuel door), main fuel line, and return. Do I just need to add a T or Y connector on the main fuel line, and run the second tank line to that T? Do I need both returns hooked up, or just the one to the main tank?

I have considered a bed mount aux tank but I just can't spare the extra space in the bed. I have the 6.5' box with two toolboxes and the fifth wheel hitch already taking up most usable space.
 

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At minimum, you need to have supply and return to EACH tank. Running on one tank and returning to the other will cause an overfill on one, and empty fast on the other. If you tee in, you will need check valves for each run to prevent them from crossing back over.
 
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