I have the Tusk cap also and thought it was a leaker - you just gotta really tighten it down - last 1/4 turn with both hands. over time it is not so difficult to get this tight and it does not leak. The gasket must get softer with use.
BE CAREFUL when putting the Tusk cap on. It is easy to start "mis threading" the cap and the aluminum will shred the plastic tank. I started to do this but caught it before I ruined the tank threads. Now I am super careful that the cap is in proper alignment before starting to torque it down.