Howdy and welcome to Airforums.com!
You have found the right place for your question, but as you can see, more information would be helpful to answer it.
I would add one obvious thing though to begin with: the most common place that will leak (and when it does, water can find itself in many different places), is the three roof vents for the plumbing system that are on your roof.
Have a look at this thread
to get a clear picture of waht needs to be done.
It's relatively easy to do that job, and they can go on a regular basis, so it's great place to start. But the important thing to get to first is the roof, windows and doors, because if your trailer leaks, it will never be a fun place to camp in.
And let's have some photos! We loooove photos..