I installed my RDS 60 gallon aux tank/tool box combo when I first got my truck. It is gravity feed, fills much faster than the truck burns fuel, and never had a CEL. The replacement tanks sound nice, but you are spending a lot of $$$ for another 15 gallon capacity which just wasn't worth it for me.
Lots of folks on this site will try to talk you out of an aux tank. I feel my RDS was one of the best mods I have done on the truck. Having extra fuel capacity doesn't mean you can't stop when you want to. But it does give you the ability to choose where you stop. With 96 gallon capacity, I never have to stop for fuel during our typical weekend trips. On longer trips we can stop at rest stops to use the restroom, walk the dogs, take a break from driving. Much nicer to do that than be forced to stop at a truck stop.