Water Tank in Caravan

NaH they don't need a plug theres a breather in there for filling.

Cheers.

Normally they are fitted with a plug to stop insects, etc getting in. The tank has a separate air breather so that air can enter when you are pumping water out of the tank, either manually or using an electric pump.

Thanks for such quick response ppl ! Is appreciated

To make a plug for the filler neck buy a plastic chair foot the correct size to be a snug fit and screw a stainless self tapper about 50mm long into the centre of it so you have a handle to remove it when necessary.
Using corks or rubber plugs they tend to soften and stick over time where the plastic chair foot works well.