Sounds like the contact is stuck in the on position. Will the press button switch disassemble from the body of the unit, might only need a clean up or pos a new switch..Mick
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The membrane type switches are prone to shorting and effectively becoming stuck on; a fairly easy repair.
Exploded diagram showing part numbers available here. Most accessory stores would carry the switch/membrane, just check the part number off the drawing.
Repair instructions available on this page at Thetford, click the top option for sanitation products. The manual is about 10mb, but covers all common repairs.