I've got an old Thetford Cassette Porta-Pottie in my 1998 StarCraft Spacemaster. I have not been able to flush it for quite some time. I have checked all fuses in the camper and all is well. This is the only electric accessory that does not work. I'm getting no motor sound or anything when I try to flush, so I'm assuming it's an electrical problem. I've got water in the cassette and in the upper chamber where the flush water is supposed to come from. Is there a safety switch to make sure the cassette is installed before allowing the flush? Any ideas how to trouble shoot and find out why it won't flush??