Why is it many don't say the make and model of the toilet?
If its a Thetford then go to www.thetford-europe.com service then repair instuctins then down load the first pdf on sanitation one pdf covers all models. If you still can't get it going drop them an email ....look for 'Ask Thetford' in the menu list. If fuse ok then a test for a jammed pump....turn 12volt light in the toilet press toilet flush then see if light dims if it does then pump is knackered or jammed. If the latter then remove the pump turn it bottom up ad if you get a small screwdriver or something to turn the impeller in the bottom of the pump until if feels free then place pump in water (don't try it dry) see if it works if not replace pump. If no dim of the lights and fuse is ok then it could be microswitch problem refer to repairs pdf.
Another thing to look for if you can hear the pump work is the flush tube has come adrift.
