I have been using an auxiliary tank for 20 years.
My tank flows by gravity to the truck main tank through the fill tube.
I had to add a second line from the top of the fill tube to the top of my auxiliary tank to allow the air from the truck tank to vent off to the top of the auxiliary tank. The 2 tanks are connected as one and the truck filler cap is the only venting required. The auxiliary tank has a non vented cap. So no fumes collected above the fuel filler.
It takes about on hour to dump 22 gallons in the main tank when truck running or not.
I looked at the available auxiliary tank kits and to me they will not work properly. And using a pump will pressurise the truck tank and vent out the filler cap while the auxiliary tank will suck air to fill the void.
My system simply vent the truck tank in the auxiliary in a closed system.
Barbara and Laurent, Hartland Big Country 3500RL. 39 ft long and 15500 GVW.
2005 Ford F250 SD, XL F250 4x4, Long Box, 6.0L Diesel, 6 Speed Stick, Hypertech Max Energy for Fuel mileage of 21 MPusG empty, 12.6 MPusG pulling the BC. ScangaugeII for display..