multiple flushes. dump, fill with clean water to almost completely full, dump, fill again almost full, dump, repeat about 3 - 4 times. If still not reading properly, fill about 2/3 with clean water, put in some cleaner (I think people recommend calgon?), take the trailer on a drive to slosh the clean water all around and help get any goo that's stuck on the walls to loosen up. Then return, dump, fill, dump a couple more times, and hopefully your sensors will be clean. When I'm packing up after a trip, I make sure I dump/fill/dump/fill at least 3 times before I unhook everything. Of course, make sure you aren't in a drought area or somewhere with restricted water use, as you are going to be wasting a lot of water.