There are some really cool tank monitor systems out there. Most of them can be installed by your normal average reasonably handy RV owner. I do intend to get that done on my
Scamp one of these days. I've got one I've bought. Mostly aimed at the potable water side.
But right now I just use memory and math. I have a 10 gallon potable water tank. Well a little more than 10 gallons is my experience but more or less than gallons. My gray water is 26 gallons. Now mostly I dry camp. Even if I'm in an RV park I just use my tank and then I have an adapter that makes it much easier to fill it from the available water. I only hooked directly up to "city water" if I really am going to be using a bunch of water and have full hookups and can essentially take my gray water connected to the gray water at the campsite and open the valve and leave it open.
The rest of the time I keep track of the number of times I've filled my potable water tank. And essentially empty the gray water a little after emptying my second filling of my potable water.
With the stamp you have that gray water pump filter for the shower. And I have a whole lot more trouble with that than I do my gray water tank filling up.