Dan, you may try rinsing the tank out a couple of times, if you do not have a black tank flush device, just keep flushing until the tank is full. Then drain. After doing that a couple of time fill the tank, add a cup of liq fabric softner and a cup of liq laundry soap. Then drag it around for 10 - 15 miles and let it slosh around abit. The fabric softner will cause any old paper to "turn lose" it may be covering the sensor. Flush it out again and that may clear it up for you. Hope this helps.
George & Julie
06 Diplomat PDQ