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I had a similar problem with mine (didn't have the key for it, and it was a different lock than every other hatch on the trailer). So I pulled out the gaucho that the water fill is behind, and I was able (with minimal brute force) to remove the "latch" from inside, which then allowed me to remove the locking assembly, and to replace the whole shebang.
All of this, was, of course a total waste of time, as that water fill door is very poorly designed and allows water running down the side of the trailer to funnel down into your fresh water tank. You might wonder just how much water could possibly get past the fill door, and I tell ya, its a lot. I had a bucket under the fill inlet inside the trailer during my renovation, and it would easily collect a gallon of water during a moderate rainstorm. Since the plastic "funnel" shaped thing was cracked anyway, I eventually swapped the whole assembly out for a modern stainless steel number with a screw cap and a gasket around the door. No more nasty run-off in my freshwater tank now.