Another half-baked idea is one pulley suspended from the two racks right at the edge of the roof. This would need a ladder of just the right length to act as a ramp of sorts to avoid scraping the side of the camper. You would hoist it right up to the edge, lock it off with a yachting or climbing cam lock, climb up the ladder to push it over the edge onto the rack and secure it.
If it was me I would have those racks much further apart otherwise I would worry greatly if the bow wasn’t tied off and that would entail more fussing.
This gives me a potentially easy low tech idea — just tie a rope to the bow that goes through the front rack and use that to pull/steady the boat as I walk it up a ladder on the back of the camper.
Tying the ends of the boat will be no problem — no way I’d want to transport anything this long any other way. I currently just clip carabiners on small pull lines off the bow and the stern and they’re held nice and firm with a lines with ratchets.
The only issue with this is it requires a lot of ladder moving — up the side to run the rope, up the back to move the boat, and two more positions along the side to secure the boat and secure the tie down line off the bow.