I had not seen this earlier but having a 21RB BF I know what you are talking about.
I ended up installing a new water fill port and hose due to problems. I supported the hose, and it actually goes uphill and then back down hill to the tank. This is a terrible installation the way BF built it. You need a clear plastic 1/2 ID hose about 2 ft long with a brass hose fitting on it, I made it, so the clear hose reaches all the way into the tank. I open the cargo door and watch the water rise up in the tank (some trailers have a panel that blocks view of the aft most portion of the tank.)
The owners manual shows an adapter similar to this, or the one above that Dave illustrates and requires you use it to allow air to vent properly. I found out the hard way that shoving a garden hose end in the filler is a recipe for disaster and THEN I read the owners manual. Then I fixed the crappy installation BF did on this trailer.
If you are a member of BOCI I did a complete thread on there.
I'll attach a couple of pics.
First is the clear hose with brass hose adapter. It does two things, it gets the water all the way to the tank, and it is small enough to allow air to escape from the tank during filling. DO NOT depend on the vent to allow the air to escape during filling. It is small, probably has water standing in it, and probably has mold in it too.
My fill tube was too short and poorly fitted to the outside filler neck, It came off as soon as the weight
of water got in it and also the air could not escape. I managed to clean it up fast and none got under the floor. I replace the filler neck as the vent nipple broke, and then installed a new longer vent line and new longer filler hose, and supported them with straps. It was a real pita thats for sure but now I don't have water sloshing out and water does not stand in a sagging filler hose either.