Mine's the opposite - Pull the dump valve, splush, check the gauge and now it says 2/3 full. Done.
Actually, after having lived in this unit full-time for almost two years, we've found our reliable indicators of when each tank is full:
1. Front gray tank - Random passer-by: "Hey, you have some type of leak under your RV".
2. Rear gray tank - Wife: "The kitchen sink won't drain!"
3. Black tank - Toilet: "Burrrrrp!"
For my rig the tank full indicators are:
1. Front (Kitchen) gray tank - water won't drain from the kitchen sinks
2. Rear (Shower) gray tank - Standing water in the shower
3. Black Tank (Toilet) - Backsplashing "BUURP" (as the fluid height in the black tank raises above the roof vent pipe)
I have had my grey tanks overfill several times and never experienced an overflow of fluid onto the ground. The black tank has never overflowed, either. I don't think my tanks have overflow orifices other than the rooftop vent lines.
Now living alone, the black tank burp is my signal it is time to dump all 3 tanks. A good tank dump procedure includes first dumping the black (with built-in tank washer/sprayer being used), then dumping the detergent rich kitchen gray water to wash out the plumbing lines and discharge hose, and lastly dumping the mainly clearwater shower gray tank to flush out the plumbing and hose.
Commenting on the thread title, I have a built-in sewer "RANK SENSOR" on the center of my face.
