Although it won't work with a bed cover, here's what I ended up with as far as aux tank and existing tool box. I have an extended cab with the OEM running boards that extend just ahead of the dually fenders, so I can step up and easily get into the box. I really wanted a fuel/toolbox combo, but most of the toolboxes with the shallow depth size (6" - 9") wasn't going to work for me. I did find a toolbox/ fuel tank combo that had a 18" deep tool box and 48 gallon fuel tank (RDS-72774), but I'd already committed to the tank shown.