You have a layer of crud that has built up on the walls of the tank. Most likely it is grease. Water and soap from the shower and dishwasher are cleaning the tank - at least temporarily.
You could use Happy Camper Extreme clean or other tank cleaners to try and get the tank clean. And watch the kitchen waste you dump down the drain.
Roger & Mary
2017 Winnebago Navion 24V (Sold)
2014 Tiffin Phaeton 36GH (Sold)