I have not seen a float system in any fresh or grey tanks,
What I have seen is a line of 4 probes running vertical up side wall of the tank,
The probes use the contents (water) to cause resistance between each probe thus roughly working out the contents,
I bet when it was working you would be 1/4 or 1/2 but not getting anything in between like the diesel gauge (which has float system)
I'd check the wires on the outside of the tank, if all is good there then get a few buckets of hot (hot as possible) with a good dash of bio washing liquid down plug holes, let sit for a few mins then bring van for 20minute drive on windy road to swill all that around, let sit for a few mins more and then rinse with clean water,
Might solve the problem, but if it doesn't when you open the inspection cap under the tank it should be a lot cleaner to work in,
Let us know how ye get on 
