Does anybody have any good tricks on completely cleaning out a black water tank? I have filled it up with water many times, added the treatment, drove it around to break things up, and still my tank reads 3/4 full after draining. If I hold the flusher down and push a stick down into the tank, it looks as if it is nearly empty. Do I have a bad black water guage? The camper is new and has only been used 5 times. Thanks!