[JRTJH]

The fresh water fill port is probably located on the side of your trailer and inside a cupboard or cabinet/possibly in a bench or bed frame base. As such, inside the trailer there is access to the back of the fresh water fill. That hose connects to both the back of the plastic wall fitting and to the top of the fresh water tank. You might be able to follow the hose to the hole cut in the trailer floor and "if it's threaded, rethread and tighten it" into the top of the fresh water tank. You'll probably find that it is much easier to twist the hose if you completely remove it from the back side of the plastic wall fitting. Once the tank end is secured, then reinstall the wall end.

You didn't say whether your trailer is still in warranty or not. If it is and if you don't feel up to the task, you can always contact your dealership to arrange for a repair.