I've seen this happen before.
The lens should seal to the base of the light and the base should seal to your trailer so that water doesn't leak in. If the lens fills with water, depending on the type of fixture you have, it may also leak into the wall of your unit where the wire exits the siding.
There should be a small weeping hole on the lower side of the lens for drainage. If the lens is installed up-side-down, this hole acts as a means of entry rather than a means of exit for water.
I've seen roof leaks
drip down inside walls, along wires and out into light fixtures.
If you take a piece of duct tape longer than your light is wide and attach about half its width to the wall of your unit just above the light and the remainder of the width covering the top of the light, it will shed any water running down the wall of your unit, preventing it from entering the light.
If this works the problem is with the light fixture. The lens is attached up-side-down, the lens to fixture seal is leaking, the fixture to trailer seal is leaking etc.
If the lens floods with the duct tape in place your leak is elsewhere.