I have been narrowboating for many years (like many on this site) and can say I have never tipped my Portipotti other than in an authorised disposal point.
A simple solution is to purchase a second holding tank. (easily purchased).
There have been many articles with reference to lack of/cost of emptying fixed tanks, with another solution being to do comprehensive research before purchasing, but I am very happy with my set-up.
T. Mapstore