RDS makes several tool box tank combs that are flush with the truck bed top rails. Makes a very nice unit if you don't want to carry a lot of tools. I think my tool box is about 5 inches deep with a 50 gallon tank underneath. I got mine at Northern tools.
2012 Hitchhiker Discover America 349 RSB
Bigfoot levelers, Tailgater satellite
2011 Ford F350 PSD, Pullrite Superglide
RDS Toolbox Tank Combo, Garmin GPS & an itch to hit the road
We use RDS box/tanks mounted below the rails. Experience has shown anything above the rails can come into play if you travel off level such as steep fuel center driveway entrances etc. Doesn't take much to damage the fiver cap.