Having Trouble with my water system.......

[Joshua & Lauren Adams]

1975 VW Campmobile

So one of my first projects ,while waiting for a professional opinion on our motor, is our H2O system. I have the system in pieces and I have hit a snag. I cant seem to separate the small fill pipe that connects to the top of the water tank. Seems as if a good twist should do it, however it just twists in place?? Should I just break it and start anew? Our tank needs to be flushed and cleaned.

Also any suggestions on a water pump system upgrade etc..... im redoing all the lines and such?

I can provide pics if needed.