If the waste tank is one of the modern (i.e. smaller) type, disassemble it and clean the float.
If it is the smaller type containers the entire trap assembly will be mounted on an evident circular plate that can be removed by turning - look for an arrow head and alignment marker at about 5 o'clock assuming the central point at that end of the tank to be 12. You will need to slide out the trap cover plate and put something like a piece of 3x2" timber across the cover slots to turn it - they are VERY stiff - and lift the entire trap out. The float is on the back wall of the container at about 1 o'clock and may not only be bunged up but also not in its mounts correctly.
As someone else said you can try tank cleaner first - which does work - but if the float is not in its mounts that won't help.
The main cause of the float becoming dislodged is due to excessive tank swilling end-to-end when rinsing during the emptying process.
Small addition to Woody's very good description. If you find the assembly very tight to move run a few drops of olive or veg oil into the crack (not mineral oil or WD40 etc.) let it soak in for a few minutes and the thing should then move easily. (Had this idea in mid France after toilet tank stuck in housing due to blade assembly pivots jumping out of their pivot points = reason unknown, but a messy couple of hours).
You can lubricate any part of the trap mechanism slides etc, the main seal, and any of the vent seals preferably with olive oil to keep them soft - and working!
Don't forget the anti blowback vent as well that is under the removable oval "Thetford" plate behind the top handle.
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