Valhalla:
It depends on the way the RV was designed. On our Georgetown, the long linear water seals between the RV sidewall and the slide-out roof were pretty wimpy and I assumed they were secondary seals for any water that blew under the topper. They also tended to roll over on themselves during slide movement.
After our flooding incident, I made it a point to take a look at the seals after deployment and go roll them over again if they weren't set correctly.
I always considered the toppers to be the primary defence against rain. It can often be close to horizontal around here