I had a similar setup and wanted to be able to fill directly. I just drilled a hole near the top of the tank and installed a galvanized fitting to which I attached a hose fitting. I leave my fill hose connected directly to this fitting and have found the tank fills much more quickly than it did through the factory setup. If, for some reason, I want to use the original fill valve, I just unscrew the hose from the tank and attach it to the original fitting.
In order to fill the tank from a non pressurized source, I could use (haven't yet) an old 12v SureFlo pump that I have. I attached a section of hose on each end: one that screws into my direct fill fitting or the original fitting and the other that drops into whatever water source is available. Good luck
jor
