if its a cassette toilet theres also two micro switches that can cause the non flush symptom ,replace the one under the flush knob first (the switches are available in good camping shops or by mail order)if it still doesent flush then replace the one in the cassette "holder" its self ,this one when it breaks stops it flushing because the toilette "thinks" the cassette isnt in the hole. this one is tricky to replace as you have to work on it via the door.
I had a flush that wouldn't work. My problem was the pump had seized due to winter storage. Facing the toilet, remove the paper holder on the right, it just pulls out. Underneath you will see the pump located by one screw, release this and remove pump. All I did was rotate the pump by hand and this freed it. Sounds complicated but it isn't. My caravan shop says this is quite common. Lot cheaper than buying a new pump. Did mine 2 years ago and it's still going strong. Good luck.