How to know when freshwater tank is full?????

I have been RVing since 1979. During this time, I have owned a dozen or more travel trailers and or 5th wheels RVs. Each of these RVs had a gravity freshwater fill port with a vent/overfill drain at a location that was easy to see when the freshwater tank was full. We boondocks more than 75% of the time; therefore, it is very important to have a full freshwater type.

Our current 5th wheel does not have a gravity freshwater filled port and the vent/overflow drain is located near the wheels where it is almost impossible to see when the freshwater tank is full and water overflows. I thought about running some type of hose, connected to the existing vent/overflow drain to someplace such as near the City water hose connection so I can see when the freshwater tank is full. Do any of you have a better suggestion?

I’m not trying to address the problem of siphoning water out of the freshwater tank. There are other threads which address the siphoning problem.