I have a 2014 Sundance XLT 5th wheel. After returning home from a five day camping trip I went to empty the holding tanks. At the campsite, the black tank still showed empty but after the two hour drive home, it showed full. Obviously some paper or stuff on the probe so need to get a good flush. I have my own dump station with water handy so decided to fill the tank before dumping. I used a hose through the toilet to do this. I was watching it with my hand on the shutoff (didn't want to overflow) just as the water was coming up into the toilet pipe just before I shut off the water, there was a loud "pop" sound and the water lever dropped back out of the pipe. I fully expected that something had broken and there would be water on the ground under the trailer. Wrong, nothing on the ground and nothing in the basement area. There was a large bluge in the colopast under the deep (outlet) end of the tank but that was all. Went ahead and dumped the tank and flushed it a couple of times. The tank showed empty and the bulge in the colopast went away so apparently all is well. BUT, what happened with the "pop" and the sudden drop in water level??????????