Question, did you try a new garden hose rubber gasket? They get old, dry and stiff with age.
I do not know your model. On mine which is really no help to you probably. The city water connection is along side the water heater. I can easily access it, the same way I access the backside of the water heater to flip the bypass valves. In my case, I could cut the pex tubing and spice in a new piece if I needed to. But as I recall the backside of mine has a 1/2" npt pipe fitting to the check valve/hose connection fitting. You might have the same setup, and just need the outside component. Unfortunately your rig is 13 years older than mine, so the technology maybe different