I found it rather tedious to have to stand and hold the filler in place when filling the fresh water tank.
I had the standard water fill sold at RV stores everywhere, so that seemed like a good starting point. I bought a replacement cap for the water fill opening and then drilled a hole in the center of it. I removed the plastic tube from the filler and fit the new cap onto the yellow stem of the filler. I epoxied it in place and replaced the plastic tube.
Now I merely insert the tube into the opening, twist the cover on tightly and turn on the water.
I've learned to tell when the tank is just about full: it will start to gurgle before any water starts to come back out the overflow tube.
I have since added one of those right-angle adapters so that the hose hangs straight down.