Winnebago roof is Filon sheet covering a block of foam. I'm not sure how a rack system might be secured to that material. Factory installed fixtures needing support get a mounting plate before roof is laminated together.
Looking at the roof of my Itasca, with four vent covers, three antennae, shower skylight, refrigerator vent, air conditioner and two waste-tank vents, I don't see a big enough free space for a kayak, unless it was less than eight feet and could be carried sideways to direction of travel. But it I were to carry it that way, the aluminum frame running along the sides of roof (exposed part looks like a gutter) becomes a candidate for mounting a rack across the full width. I've used gutter-mount racks successfully before, to carry a 14-foot boat, but never had one more than 80 inches across; RV would need one 100-102 inches. Most on the market today are 48-60 inches.
Most recent rack I bought was a suction cup mount with straps tying to the gutters. I last used that on a 1992 vintage pickup, but it could work on my van, which has gutters.
Tom Test
Itasca Spirit 29B