Pull the trim off the inside and look for rust. Wire brush it good, paint with a rust preventative, let dry, then put silicon caulk on it.
Pull the outer trim off the outside and do the same thing. Just be careful you don't crack the windshield. My outer trim just pulled off after removing the metal end pieces.
My rig an '05 Itasca Meridian. Winnebago product.
What I did stopped my problem but don't wait too long or the rust will require MAJOR, VERY EXPENSIVE repairs to the tune of about $4,000 from what I've read.