You may want to look at the Fold-A-Cover G4 elite.
It has the best locking latch system I've seen on a folding cover, and you can't open it with a coat hanger or Slim Jim like the Undercover or Bak.
Since no auxiliary/replacement tanks are ever likely to be available, I was thinking about going this route....http://titanfueltanks.com/titan-sidekick-15-gal-portable-liquid-tank-5040015