Boy do you folks like to do things the hard way.
There is a check valve in your fresh water pump, (that is hte pump that pulls water out of the FW tank)
Simply pick up a pair of "T" fittings, a valve and a bit of pipe and a few elbows. Unhook the water line from the pump filter inlet and install a "T", Hook it back up, now do the same on the pump outlet, Hook the two "T"s together with a valve
Valve closed, normal operation, Valve open, Bypasses the pump's check valve and allows city water to fill your FW tank.
Method 2 involves the same idea, however in this case you have a special "Fill port" on the FW tank, and you use only one "T" and route to the fill port.
Method 3 is use a water theif on the sink and run the hose out via the kitchen window into the fill port (simpliest of all) note, some sinks you don't need the water theif, a simple 1/2 inch coupler, nipple and 1/2 to garden hose connector and you can replace the sink head with a garden hose.. (this is the kind where the sink facuet is a on a flex hose that pulls out)
All very simple
A "Water theif" is a rubber adapter that slips over the sink areator and has a garden hose fitting