It would help to have a little more context. What kind of trailer is this, and what part of the trailer are we looking at? Where does the water appear?
In general, isolating a leak takes a dry day and some patience. Starting at the bottom and working your way up, apply water to likely culprits- fittings, vents, windows
, belly band seam, even exterior lights
. A garden watering can with a spout or a garden hose on low allows you to control where the water is applied. Check to see when water appears inside the trailer- it can take some time and appear at some distance from the source.