I would find a grease remover at a RV store, or supermarket. Let it sit for a few days and then drive around to loosen thing up. The vent may not be the problem.
So what happened when you tried to fill using a bucket to measure?
If you left it open for 3 days, even if partially blocked, any water should have been able to dribble out if it was a partial blockage.
Sounds like youe sensors are dirty. Buy the cheapest liquid dish washer detergent you can find, before next trip add 5 gallons of hot water and a cup of the detergent. The action of riding down the road will clean the sensors. Also helps if the gray tank starts to smell.
Glad you said you are a New RVer. We gave up trusting the gages years ago. Mostly there as a make you feel good thing. Mine read full after emptying and I knew they were empty. Simplest thing to do with ether gray or black tanks... key word "simplest". Dump 5 to 10 gals of water, half bottle of dawn into each tank. Hookup and drive about 5 miles OR do just before your next trip and at your first over-night dump. And don't be surprised your sensor still reads 2/3rd. The sky is not falling, just cheap sensors.
Ask around and see if anyone near you has a digital camera inspection camera. If not, cheap models at Harbor Freight go for less than $100. Kind of handy for more than RV use.
And the tank monster will form from the mass and take over the earth...no, you aren't going to fill 1/3 of your grey tank with grease and hair. Even 1 inch on the bottom would be a lot.
As others have suggested, filling with a measured quantity of water after dumping will tell you if it's empty or not.
New RVer here. My Grey water tank will only empty down to the 2/3rd level. Not completely clogged, obviously, but why would it stop draining at that point? I don't think it's a sensor issue because it does seem to stop short of completely emptying, plus it doesn't take long to fill back up to the full mark.
Thanks for the quick replies, everyone. I'm never sure of anything, when it comes to the tank senors. Lol
My impression is, it takes less time to drain than it used to. It's like, "Wow. Done already?" That, plus it only takes a day to reach Full again (normal water usage), rather than the 3 days it used to take, after emptying the tank. It's a 52 gallon tank. Just doesn't make sense.
I've left the valve open for several days, and it never goes below 2/3rds full. It doesn't go back to full until I close the valve. I'm hoping the next time we drive it, maybe something will work itself loose and the problem will go away.
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