I have done auto glass for many years and would like to give you a little advice on finding and fixing leaks.
First, use a hose to check for leaks and when you find it, fix it from the outside. When you try to fix from the inside the water will just migrate somewhere else and still come in.
Start low on the trailer with the hose and gradually move your way up. If you just start running water all over the top, its very hard to discover the source. When you get to the top, hold the hose to one side and then the other. You are going to need someone inside helping with a flashlight or they can run the hose with you inside.
I honestly don't know the best product for a trailer, because its different materials than a car. I would ask here or a local trailer/camper sales place. When you find out what kind of sealant to use and get your leak fixed, share it here so we will have that info.
Hope this can help in some way.
Randy