I don't think the post should have said "all class C's will leak" I think it should have said "All Class C's have the potential to leak" just like all other RV's.
Years ago someone posted "imagine that your stick house is subjected to a major earthquake everyday, that is what your RV goes through when you are driving. That is so true, even a basic class B with only a roof vent could leak. In fact I know cars and SUV's that have developed road noise and leaks around the doors.
Like everything keep up with maintenance and hopefully you will be ahead of any leaks.