I need a way to lock my fresh water inlet without having to cut a new hole for a locking door over the water inlet. Any ideas?
It's really not difficult or expensive to install a locking water door.
But if you like you could install an appropriately sized plastic ball valve just downstream of the filler opening inside the trailer. then open it only when adding water.
Another Idea would be to simply drill a small hole in the lip and lid of your present filler door then get a tiny padlock for locking it. It would be like a pierced ear and add a touch of attitude to your little trailer.