Grey water tank won't drain completely

Are you dumping at the same location? Trailer not level or leaning away from the dump valves?

We all suspect sensors because we have all been there and grey tanks rarely clog. That said something could be caught and not letting it drain so be a bit more scientific with "seems like fills faster". Keep track as best you can over the next use age period.

The problem is not the tank draining it is that the sensors always show 2/3.

And that is normal.. Alas there is no effective cure for it other than a different type of sensor

HORST miracle probes claim to work better.

See Level sensors (not probes) do work better.

When the grey tank valve is open, have someone dump a gallon of water in to the kitchen sink. If the water quickly flows out of the drain, then that will show that there is not something blocking the drain and that the most likely cause is a bad sensor.

I would also think that it is much more likely this is a sensor issue versus anything that would allow 2/3rd to drain and somehow block the last 1/3rd.

Are you sure it's not the sensor instead of the tank actually draining? Try flushing the tank a few times (Fill completely and then empty). This might clean the sensors. All kinds of things can stick to the probes and cause a mis-reading. Frankly the stock sensors aren't really very reliable. One of the reasons I installed tank flush systems on both black and grey.