The gray tank could be full of soap suds, but still empty of liquid. Try this test, just for giggles… When you've pumped the gray tank empty and it still shows 1/3 or 2/3 full on the so-called gauge, fill it again, one gallon at a time using a one-gallon bucket filled from the shower nozzle, and emptied down the shower drain. See how many gallons it takes, and compare that to what the owner's manual says your gray tank capacity is.
What this little test will do is show you that the pump really does pump the tank empty, and show you that the tank gauge lies like a politician in an election year.
Engineering: Finding complex solutions to simple problems you didn't even know you had.