Diesel - yes ?
I also have an RDS tank with the gravity flow system. They sell an install kit that includes the check valve which stops back flow. There is also a manual on/off valve on the tank. I believe in KISS.
I got the 40 gallon tank/tool box combo which fits below the rails so I can also have a bed cover. If I was doing it over I would get the 60 gallon (?) tank with the smaller tool box. It has the same footprint.
Northern Tool sells them. I suggest going to the RDS site to view all the possible tanks (there are lots). I have even read they will custom make one but have not confirmed that.