I don't have a black water tank, but here's a little trick I use to get approximately how full the gray water tank is. All the water I use is from the fresh water tank, 12 gallons. The gray water tank is 26 gallons. Therefore I know I can fill the fresh water tank more than twice before I have to worry about the gray water tank.
I refill when stopped for long periods with a 3 gallon jug. I keep track of how many times I pour 3 gallons into the fresh water tank, therefore have approximate level or less than level in gray water tank.
It's like any other usage item, propane
, water, battery
, etc., etc., after you've done it a bit you get a pretty good idea of when to refill propane
, recharge batteries, empty gray water, fill fresh water. I can't set a specific time or interval because it depends on many variables, is it colder or warmer, have we eaten out more or less than normal, is it hot so I drink more water, is it cold so I drink more coffee, and the list goes on and on.
I've missed once on the battery
, and once on the propane
. Those misses taught me a bit more about managing those things.
I have to admit that when it comes time to replace the propane tank I'll get one with a built in level gage.