Sometimes the sensors inside of the tanks that sense something in there, get covered up with toilet paper or other debris; that will give you a false reading. I would fill the tanks until you know they are full with fresh water, and then drive the trailer to a local campground with a dump station hookup. Drain the tank,(you will hear and feel the weight of the water draining through the pipe into their tank. Then check your sensors to verify they are empty. As a helpful hint, I put 2 capfuls of Calgon water softener liquid- available at most any grocery store for laundry- to help make the inside of the tank "slick", therefore nothing sticks to it. Also helps to cut down foul odors, too.
Hope this helps you! Randy
/SIGPIC]'08 V-lite Flagstaff 30WRLS
'06 Ram 1500 QC hemi
Reese dual cam sway control,
K&N series 77 intake, Hellwig helper spgs. LT tires,
Flowmaster "true duals", 380 h.p., Bilstein shocks