These Killian screw caps are for our large-mouth water containers. The solid top cap has an 'O' ring seal inside. The other has a clear plastic insert with 2 holes (one to be enlarged for your water/waste pipe and the other an air hole). The internal diameter is approx 95mm.
Use a sharp Stanley knife to enlarge the centre hole in the clear 'Lid Insert' to create a snug fitting opening for your waste pipe or fresh water pipe/pump cable. This will stop any water sloshing out as you drive when the tank is full and also holds the submersible pump at the correct height in your fresh water tank.
When installing your water system, allow enough pipe length so that you can easily remove the tank from it's cupboard far enough to make taking the cap off easy. Don't forget that a submersible pump will leak the contents of your pipe on the floor if allowed to hang down vertically once removed from the tank.