Try adding the above to your tank to really flush it out or some dish/laundry soap. Fill the tank partially and then drive around to allow the water to slosh around the tank then dump.
If that doesn't work you may want to try to recalibrate your grey water tank since you have indicated you have flushed it and it is indeed empty.
I purchase this water meter to help me determine how much water I'm putting into the tanks for calibration
You may want to try to recalibrate your grey water tank.
This is my procedure for calibrating one tank since I can't seem to understand the Micropulse instructions for recalibrating one tank.
1. Verify your tank is indeed empty.
2. Verify your batteries are fully charged. Take the trailer off shore power. My monitor panel doesn't seem to want to calibrate if on shore power and the voltage is 13+ volts.
3. In the water pump area you should find the telephone wires that connect to the tanks. In my 25fb there is a black wire for the black tank, gray for the gray tank and white for the fresh water tank. Each wire is connected to the sensor wire with a telephone line coupler. Unplug the white and black wires. This will isolate those tanks during the Micropulse calibration sequence.
4. Using the Micropulse instructions put the monitor panel into calibration mode and press the Status button until Ready for Empty light is lit.
5. Run your water hose to your kitchen sink and connect the water meter and run 5 gallons of water down the sink. The tank should have enough water to cover the sensor. I usually wait 10 minutes before going to the next step.
6. Press Status button to move to Ready for Full
7. Run the water hose and put in an additional 30 gallons. Your gray tank should be 37 gallons capacity so you should be able to hold 35 gallons. Wait 10 minutes
8. Press the Status button again and finish the calibration. Turn off the monitor panel and then turn it on. It should read full. Turn off the panel
9. Dump the gray water. Turn on the panel and hopefully it will read empty. If you want to test everything then put about 15 gallons and see if the panel reads 1/2 full
10. Don't forget to reconnect your other holding tank sensors after you finish.