I would be surprised if anyone on this forum used the roof racks at all. For one thing the roof is a very important part of the RV. It is also very delicate. Look at it sideways and you will develop a leak. Once a leak starts is like a cancer and being on top of the RV it will follow the laws of gravity. Dripping down down and down till it reaches the pavement. All through the camper, if left alone it will rot the entire camper. You can have this leak without even knowing about it. So why chance putting something up there that can compromise the integrity of the roof..
Not only that it looks funny when you tie something down up there. Like a box or tarp or anything really... Leave the roof empty you will be very happy you did.