Waste water tank blocked up

My waste water tank is getting slower by the day to empty. Might be a build up of fatty deposits.

Does anyone have an answer to this.

By the way it has a large diameter outlet pipe but the gunge is most probably in the tank itself and as the tank holds around 130 litres and we use the motohome often we just want a good cheap all round solution, get it, to the problem.

David