sounds like pumps jamed find it turn it upside down and turn the impeller if you can see it slowly with a small screwdriver either way until if feels free. replace try don't try when its out of the water as it will damage the pump. Alterantive it may need replacing by lights dimming you know current is reaching the pump. follow this for repair inst there free
Thetford......www.thetford-europe.com
www.thetford.com.au for Australian campers.
Access no matter which for info.
www.thetford-europe.com
on home page click on ‘Service’
next page click on repair instructions
next page click on sanitation. ( worth doing fridge also)
down load pdf the repair instructions covers all look up what referes to yours.
Go back a page and down load in turn all you think you might need.
bob lunar solar owner, June 2012