Found this port on my 2018 Phaeton.

[RandomDV8]

I have this locked "fuel" type door on my Phaeton and I don't know what it's for. See pics. My guess is that it's water but why is it locked and on the passenger side opposite the wet bay. Could it be DEF? But again it's on the wrong side and far from my DEF tank.

Thanks

[Skip426]

Looks like, from the small vent hole under the large opening to be a ; gravity fill for the fresh water tank .

Locked as are most, on trailers to prevent unauthorized access .

[DRM901]

It should be for city water, when you don't want to fill/use your fresh water tank. At least it is on our 2014 Allegro Bus. I'm assuming it is locked some someone can't easily contaminate you water.

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[BobGed]

Assuming it's located on the panel shown in the first photo, it is simply another way to fill your fresh water tank. The additional photos show what it looks like on the other side of the panel.

[tjvogtiii]

It is the fresh water fill for the passenger side. I use it at my home since my hose won't reach the wet bat.

[RandomDV8]

Thanks everyone