I have the 60 gallon RDS tank / tool box combo. I love it. For me (like others posted) it isn't about saving $$ per fill up, but about filling up when I want to. On weekends, I do think it saves me time as I can go round trip out to the lake and back without wasting 30 minutes of time at the fuel station. To me, that is a big deal.
I would rather have the Titan replacement, but an additional 15 gallons wasn't worth the cost they wanted. My RDS was less than $800, has a manual shut off that does not over fill the tank if left on, no CELs, no problems with the factory fuel gauge, etc.
I am glad I bought it.