I can't say for certain about the Northern Lite brand, but most molded fiberglass units like that (Scamp, Casita, etc.) join the halves with more fiberglass. They glass over the seam, and the resulting joint becomes even stronger than the rest of the shell. That said, I have seen pictures of some Trillium trailer seams that were inadequately glassed over and were leaking.
What did you use to reseal the window? Silicone is to be avoided, because when it fails it leaves a film that new silicone will not adhere to. The film (often invisible) must be removed thoroughly (a combination of acetone and abrasion) from all surfaces before new sealant is applied, and the best sealant for that application is butyl tape.