I'm also a tank only user... I'm only a long weekend camper, so no big deal.. Been doing this for years and we go to a lot of campsites that have water hookups.
I fill my 30 gallon tank with my water from home, that I know the quality of, and usually have water still in it.
Works for me and since I have a 30 gallon black tank and a 30 gallon grey tank, I know they won't fill up, no matter how much water I use!
Turn the pump off whenever we don't need water and that's what's worked for the past 10+ years I've been doing this.
I've turned off a campers water when they had water flowing out of their rig and they were gone... I was just walking around the camp ground and saw it.. Knocked on their door, no answer.. So, just turned off the water.
They came back about an hour later and wondered what happened. I came over and explained what I saw and what I did and they spent the rest of the day trying to clean it all up. The camp host had a wet/dry vac, so that really helped I'm sure..
I was already doing my tank only thing at that point, and that justified it even more..
Good luck!
Mitch