I have a similar problem on our 5er. In this case, the RV fresh tank fill hose runs horizontal for a few feet which allows some water to feed back out the inlet. I had the "Camco E-Z Fill Water Tank Filler" and replaced the included short hose with 4 feet of same type and diameter hose obtained from the local Home Depot. It's fairly easy to push this modified supply hose inside the RV fill hose far enough to get past the "hill" and fill the tank. If it does bind (from entering a sharp bend, for example), back off and rotate slightly and try again, brute force could disconnect RV hose from the inlet. It would be easy to make a "homemade" version of this (garden hose connector, hose, hose clamp).
Alternatively, I've seen someone who used a small (16-32oz.) hose end garden sprayer (with a hose adapter on both ends) with rate dial (typically used for fertilizer delivery). This can be used "inline" to the city water connection. Fill the container with bleach and it will dispense evenly at whatever rate you set. Anyway, the idea should work, something similar may be available. (DISCLAIMER:
Purchase new, wash, and never let insecticide or fertilizer residue come in contact with your fresh water. -- Just in case anyone is running to the garage right now.