You didn't mention the coach brand or year and model. Some water gauge systems are better than others, . One of the better ones is the See_Level Gauge
, and it can be retrofitted to most RVs if you have one of the other types.
Mostly, though, it is a matter of watching your use and estimating. Even if you had a dead-accurate gauge, empty is still empty and you have to go find more. If somebody takes a too-lengthy shower, knowing he used 17.2 gallons doesn't helkp much either!
We can run a week or more on 100 gallons of fresh water, when we choose to conserve our use. Maybe 4 days if we are somewhat more lavish, but even then we usually run out of gray tank space before the fresh is gone.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL