The float that has a magnet in its mount will either be stuck in the up position or will have come slightly out of its mount. In either case it cannot float on top of the poo.
Remove the (empty) waste container, open the trap and shine torch into the stern end. You will see the float - it will align with the reed relay on the PCB at the back of the waste container housing in the caravan.
If you cannot reach it look at the surroundings of the trap on top of the container where there should be a visible circle in the case. Remove the trap slide cover, get a piece of 2x2 or 3x2 or similar about 18" long, place it across the trap slide guides and turn it carefully but with some energy about 30deg anti-clockwise, then lift the trap assembly out. It is a good idea to have some newspaper handy on which to stand it as it will be covered in s**t coloured scale - and wear rubber or vinyl gloves. Reseat the float and put the whole lot back together. Job done.