I have a 50 gallon fuel tank/tool box in my truck that is flush with the top of the bed. I installed it about 6 years ago and it has worked well. It is plumbed to fill the main tank by gravity. The connection from the bottom of the tank goes directly to a fitting I installed in the filler neck. I did have to install an additional fitting in the bottom of the tank and cut a 1" hole on the bed.
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