@pip.t - sorry to take a while replying, but I only got my Wellhouse Terrier a week ago
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The water and waste tanks are slung under the driver's side, between the two wheels - fresh water near the front, and waste water at the rear. The fresh water fills via a lockable pipe in the driver's door well (so internal, and away from road dirt etc.), and both tanks have external drainage taps. Mine has the winter kit fitted, which means that the tanks are lagged and contain small heater elements.
I haven't used them in anger yet, so I can't say how well they all work in practice, sorry (I don't anticipate any issues with them though, but as a caravanner I'm used to fiddling with water fillers and tanks lol).