Leaking drain plug

I’m thinking Does the drain plug just need a rubber washer to seal it? But Coachman customer service people will have far better ideas than me. What did they say when you spoke to them? 

Many thanks I’ve not spoken to them just but I will. The drain plug comes completely out of the tank and there is no rubber washer on it, as you suggest it may just need one.

I thought of this thread just now as I drained my Coachman's onboard tank .... briefly used the caravan while we had work done in our kitchen. I fully removed the T handled drain plug for a look-see, the end just has a point & a slight reduction in diameter for the last inch or so. You can see right though to the floor while the drain plug is removed so I guess that the plug is just a push fit into a grommet of some kind that is fitted to the bottom of the tank. 

I'd guess that the grommet on yours has been dislodged somehow 🤷‍♂️ I would think that the tank needs to be fully removed to access it. I can see two retaining/support straps but that's as far as I've looked ..... 😉