They are notorious for being so inaccurate to the point of being useless. As said many times get a clear extension (45 or 90 degree or straight) to add directly to the rv then attach your hose so that you can see that they are in fact empty when draining. You can use tank cleaning chemicals, ice cubes, Geo Method (Google that) or any other magic potion & they may work once or twice. When parked hooked to sewer you can leave the gray valve open til a day or two before draining the black so you'll have a good rinse for your hose, NEVER leave the black valve open til time to dump & give it rinse til it runs fairly clear water.
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless