We also have a 2005 Itasca with the external truelevel sensors, and they give us incorrect readings, too. The black is't too troublesome; when empty, it often still says 1/3. Black readings increase as you use the facilities. The grey, on the other hand, when empty can say anything but empty...sometimes it reads 1/3, sometimes 2/3, sometimes even full right after various dumps. So we are just careful about water usage and dump each time we relocate to a new location (typically every two weeks; we full time, so tanks are constantly in use)
Sensors seem tightly attached to grey tank exterior last I checked, though your post will cause me to check them again. I have tried various cleaning methods for the grey tank (including two method and commercial products) though I do not really think it is very dirty; I think the sensors are misreading the levels.
We manage OK timing our dumps, and our grey tank sensor is a running joke between spouse and me. Still, it would be nice if it worked better.
If by chance you find a great solution for this brand of sensor, please post it. If I miraculously find a solution, I'll share it, too.