When towing with a full water tank I would get water inside the water fill hatch door that was splashing up through the fill tube even though it was capped and through the vent tube when the trailer was bouncing around when being towed. The water would then run through a slot cut in the water hatch door assembly where the cam lock would catch when you lock the door. When it ran through the slot it would leak into the wall.
I went to Lowe's or Home Depot and bought some tiny corks and a solid black rubber stopper. You can find them in the specialty hardware drawers in the section with the nuts and bolts. Ace hardware has them too.
I use a cork stuffed into the vent hose hole. I picked a stopper that would fit tight inside the hole the fill tube is connected to. I drilled through the stopper and installed a small picture hanging D-ring with wire lanyard using a stainless machine screw, stainless lock nut and 2 stainless washers. I wanted a lanyard in case I pushed it in too far I could pull it back out. I used the nut and bolt so they didn't have to worry about ripping a screw out of the stopper when pulling on the wire.
When I get to camp I pull the stopper and the Cork and put the cap back on the water fill tube. I keep the stopper and corks in the silverware drawer so I don't lose them.