DrewE explains it well. Delamination "can" spread, but if the source of the leak has been repaired, and the structural damage contained, then it can also be controlled without further damage. Ours has not spread since we've owned it, but again, it doesn't mean that your potential trailer's delam won't.... All things being unknown, and considering you are planning to be "full-timing" for a while, like others suggest it probably is best to walk away from this one and find one that does not show signs of water damage.
