My recent experience with diesel heaters says that you should keep the fuel line to the heater as short as possible, use hard plastic tubing with rubber fittings, not flexible tubing, and orient your pump at a 45 degree angle with the outlet on top. There are several kits on Amazon to allow you to drill into a tank and place a pick-up tube for diesel heaters. Most of them I have seen were being tapped into portable containers, but I'm sure they'd work for a vehicle tank as well.
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