I know there are bed mounted tanks available. We have a couple of unit members that have them. The exit form the bed mounted tank goes into the OEM tank. You fill the bed mounted one only. I have a Friend in Ocala that only buys fuel in GA. He can get there, get back, run around here and get back before he uses it all.
The biggest drawback on the newer trucks is the computers cannot deal with the tank being full for hundreds of miles, so you would have a cut off valve between the two. Fill them both, close the valve, run for a while then open the valve to refill the OEM tank.
One other option is to replace the OEM tank with a bigger one if possible.
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