I weigh about 155lbs and have allways placed bikes, canoes etc. (so the
total weight of me and luggage is often over 200lbs) on the roof by
climbing onto it and have never had a problem (four Westys over 10
years).  I do not step in the center of course.  The roof is very well
built in  my experience and I doubt you would have any problems. Be
carefull not to slip off though - I haven't yet but its slippery when
wet and I have come close!
David in OR