I have a 2014 Itasca sunstar with 2 black tanks. One started showing 3/4 full for no reason. I emptied and ran the flush for several minutes and then it showed empty as it should, now after 3-4 hours ...
Those gauges are notoriously unreliable. It's either condensation or debris on the sensor. Flushing and some home made remedies seem to help. My last MH was the worst, no matter how much I flushed or put cleaner in, the grey almost always read 2/3. In the new MH I installed tank flush on both tanks and I run the flush on every trip. Seems to have helped and they usually read correctly. There are after market sensors and levels you can buy. Depends on how much it bugs you.