I would stay away from putting the fill tube and vent in the trailer. Metal trailers are one thing, but wood is a different deal. Also be sure it is high enough up. You can purchase the RV tube and vent at any RV place. I would try to get it about two feet above the floor or the top of the tank. On my original trailer I did not get it high enough and on steep hills water leaked out the vent tube.. You can get locking doors for them to. Not the securest deal for sure, but have never heard of anyone messing with them.
Coming along nicely sir. I am anxious to hear about the first trip. When is that planned?