Well, I'd done a lot of work on the trailer, but Dad hadn't had a chance yet. Of course, the day we tied down the water tank was also the day we filled it for the first time for the new setup. Problem: wouldn't fill! Gravity just wouldn't take the water horizontally into the tank. I wish I'd have looked online first. I would have found this neat do-dad that I suspect would have solve our problems in a pinch. Instead, Dad thought the hole saw would be a better solution. I didn't have the wherewithall to stop him (never have), so this is what I got. The new hole is high enough that the water actually feeds into the tank, but the hole doesn't please me in any way. He's got a cap in mind for the hole and I DO hope we get it together this week before we head out. Otherwise, well, I suppose it'll be duct tape. Or...hmmm....I wonder if I can fiberglass patch it this week....hmmmm....

That picture looks like an ad for the old adage, "Measure twice, cut once."
I'm so afraid to cut a hole in our shell that I measure twenty times before even approaching the trailer with an implement of destruction.
There are some small doors available that might well cover your new vent as well as incorporate the filler. Here's a shot of the one I put in our 5500.
I've seen various configurations of thew same idea.

HTH
Doug