anyone got tips on getting the sensors working again?....just bought a georgie boy and the holding tank , gray and black water, aren't working properly...someone suggested a bag of ice in the black water tank to slosh around while driving.....it may have helped somewhat, as the full and 2/3 light went out, but still showing the one third light when empty and with grey water tank, all sensor lights are on when empty.....are they difficult to replace?? expensive?? any ideas??