Year of trailer and/or picture would help. Different years different windows
If your window has weep holes on the bottom of the window on the outside, you might want to make sure they did not get clogged. If your window "suddenly" started leaking at the bottom, could be overflow from track if your weep holes became clogged.
Don't forget water flows down, sometimes through hidden areas like behind frame or wall covering, where you see water may not be where it got in.
You might try using a watering can or plastic cup and bucket to figure out where it's leaking. Do a binary search, slowly water bottom left half, then bottom right half, then repeat on top left/right until the leak is found. Should be able to track down which "half" is leaking, then zero in on source by splitting that area in half and watering one half at a time.
No point in caulking or seal replacement unless you know that what you are caulking or sealing is where it leaks