Well we've just finished re-sealing the windows on our 23.5' HT 5th wheel, using butyl tape.
I can only say that I'm so glad I went ahead and did this as I can't imagine the foam seal that Jayco used could have lasted very long.
Like everyone on here said it wasn't very difficult, quite easy actually. When we took out the first window my wife said they don't even have a seal in this one, that alone should be an indicator of just how thin the seal they used compressed to. My thinking is that they use the foam seal mainly because it would be just a lot quicker to apply not because it could possibly be better, their application by the way left a lot to be desired as it wasn't applied very evenly at all.
Thanks again to everyone in this thread for the tips and ideas!