naturist wrote:
How are you determining the tank level? via the official gauges? Those are notoriously inaccurate. More so with the black than the gray, but neither are, um, really remarkably accurate/useful. And for what it is worth, the entire gray water system is designed so that when the tank gets full, it ALL backs up into the shower.
I agree with @HappyKayakers that the dump problem sounds like a cutout. When the rig makers buy the holding tanks, they have no holes in the top for input, so that they can put 'em wherever they want 'em. They drill their own with a hole saw, and often just let the cutout fall into the tank, rather than go to the trouble of fishing it out. My black tank had the cutout for the toilet in there without issue for 5-1/2 years, before it finally flushed down the drain and got stuck in the drain valve. It wasn't hard to pull on out, however.
The tanks have external gauges on the tanks so no crud on the sensors.
Once the shower backs up, the gauge reads 1/2 full, but I can take a garden hose and let it run in the kitchen sink, all the way to the point of the gauge reading full. This has no impact on the shower until the tank is truly full and then the shower will fill as it should when full.