The water level sensors in my RV's waste tanks simply don't work that well... It's possible that this is how it must be for exposed metal sensors, and I must simply live with it. Most of the time the tank reads full. I can only assume that the cause is the build-up of conductive waste material on the tank walls, and as long as this layer is even slightly damp, all the sensors are effectively shorted.
The RV forums are full of sensor cleaning remedies - from commercial products to driving with bags of ice cubes in the tank. Spray cleaning tools also abound, and while I am not in the mood to cut a huge hole in the tank to install a permanently mounted (and complex) spray system, I think there is something I can do about this. But first, I'd need to get in there!
Thus, the first order of business was finding the entry point - and it presented itself readily enough, in the form of a mechanical vent under the sink. The pipe leading to this device is entirely straight, as vents tend to be. Removing the vent allows me to insert something down the pipe directly into the tank, but what would that be? There isn't much clearance and thus I can't get my Camco rinser (40103) in there. So it needs to be flexible.
As it turns out, making one with parts I had on hand isn't so hard - a piece of garden hose, a barbed adapter to fit it, and a PVC cap that I blessed with a few small holes. And oh, it works.
PS: In the long run, I decided that I don't really care. I know how much water I had in the fresh tank, therefore I know how much water might be in the grey tank. And besides, it backs up into the shower when over-capacity