We take the simple route. As gauges never are accurate, I simply turn off the water pump, open the toilet flush valve and shine a flashlight into the tank. Believe me, if it's close you'll know it.
As for the gray tank we've found that there is a very noticeable gurgle that emanates from the shower drain before the water ever comes up into the shower. Time to empty.
Longest without emptying tanks has been ten days but we don't have washers, extra bathrooms or muddy kids any more.
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