I think Campwestcoast, has valid "general" concern, mounting on the roof would be one of those "need to be ingenious" types of things, due to the fact that once the roof leaks, it gets destroyed many many many times before it shows up inside, be it a motor home or the Hilo... It is your camper for sure, and I know the roof can support my formerly fat butt up on there.
I think Camp was pointing out what some people don't or didn't think about (or know) in general when building a solution to your problem. I think I could haul motorcycles on mine, as long as I make the load bearing mounts connect to camper in the right ways. My cycles are heavier than a canoe, my thoughts more lie in weird "loads" due to winds and such as well.
1 If I were going to make a mount, I would either copy how they mount canoes and bikes to the roof of a car, when done without drilling holes in roof and or hoping the floppy tin in the center of roof holds anything... and where they don't devalue the car by a ton, because they let the roof get all messed up...
2 And where legs/mounting brackets would be done as close to a side as reasonably possible Better yet, in fact I would probably figure out how to make the frame that the boat would rest on, actually mount on/into the sidewalls, just because leaking is a problem in campers without mounting things on the roof.
For my "in my mind design," Using angle iron to make a bracket that might not even need to me screwed to the roof, and a couple strap points added to the bottom edge of the top of campers top half (where the cables to lift hook onto), thus letting the straps hold the canoe and rubber coated or foam coated angle iron brackets to the top... after slightly more thought though I would use lag bolts and secure these angle irons to the roof from the sides, probably, so it cant wiggle in higher winds.
If you have a rubber roof model, I think it is even more imperative that you would want to mount to the sidewalls not the top, honestly.
I have a crude drawing of what I picture in my head as I explained it...
I dunno, but it is what I was thinking, that is all.