Did the neighbours give a reason why they won't pay? Maybe in their opinion the septic tank cleaning happens too often and they think their usage of toilet is less than yours?
If you need to clean the tank too often there is a reason for that. All waste water , not only toilets but also washing machine and sinks go into the tank. Imagine three families using one septic tank.... The trick is to divert all waste water into a nearby gov sewers or in the absense of that all but toilet water into the ground.
If neighbour doesn't want to pay there is not much you can do. It's not worth the legal costs and effort.