My #2 gray water tank showed full. Opened dump
valve confirmed some liquid draining through hose.
The monitor still shows the tank to be 2/3 full.
Any ideas why?

  This is usually a problem with the black water tank.  From what I discovered over the years, is that you might have some sort of build-up on the sensor.  I would try a digester and or some clorox in the system to try to clean the sensor.  Let it set for a period of time to work and if you can,  moving it will get a sloushing action to help clean.  This would br my first action.  Second it might be a faulty sensor. 
Good luck and happy travels

Thanks for the advice, this is our first trip
Just picked the trailer last Tuesday, 1500 miles
from Okeechobee FL to home in RI
Lots to learn

Seems to be a common problem, most often with black but sometimes with gray. We hardly ever check the gauges any more, we just know how long it takes us to fill the tanks. They are notoriously unreliable!

The main problems is the cheapest tank sensors are what most RV manufacturers use and they put the sensors inside the tank where they can get gunked up. That is why people recommend a good cleanout. If that doesn't fix the problem you probably have a wiring issue from the sensor to the panel. Good luck.
A few RV manufacturers put in external sensors and you can get them installed after market, these are more reliable. You will see them advertised in the small ads in the back of RV magazines with titles like "sensors that work", etc. We have external sensors and they work, Newmar installed them.

Let's see, was it in 97 or 98 that my Grey water sensors last worked?  Or, maybe that was the Black water sensors. Gee! I really don't remember when they were last accurate!

Oh well, there's no water standing in the shower, so I guess the gray water tank isn't full.

All kidding aside, these things are great when your RV is new, but unless you want to spend a lot of money on the exterior sensors, it's probably best if you do like a lot of other RVers and estimate when the tanks need to be dumped. After full-timing for awhile, you'll get pretty good at it.

I've got to agree with Jim, after a while you just know when they need dumping. Had the grey back up into the shower once, but never a problem with the black. Fred

We also just "know" when tanks need emptying.  Should say DH "knows", I don't do that part.  Wasn't there a discussion on this forum before about doing a good cleanout of the black water tank by adding Alka Seltzer with some water and driving around a while to get it moving around, then dump?  We are just getting ready to pull up stakes and travel west after being in Illinois for 2 months.  Kind of thought we might do this just for kicks but can't remember details.
Brenda

99% of the time it is the sensor as stated above.  On occasion with a new rig the drain was not properly installed.  Either the drain tube is to tall or the plastic flap made when the drain hole was drilled was not removed and now partially obstructs the drain.  Hair has also been know to drape on the sensor or worse clog the drain valve.  Take the time to fill you grey tank with water and the dump it to assure yourself that what went in came out.  Then be thankful and ignore the sensors or piddle with them to get them clean.  Solutions to the other 1% of the time are not fun to resolve.

Larry and Jacki