Stephen, (New York, NY)
Stephen, Rhino on a motorhome roof? I can understand the direction of your thinking (to a certain point), but using a product in an application for which it is not intended is simply asking for trouble of some sort down the line. Indeed I would provide some sealing to that seam, but I believe I'd reserve the Rhino for the bed of my pickup truck.
A much better solution would be to install a product from Eternabond called EternaSeam. The only preparation necessary is that the surface be clean, dry and free of any remnants of silicone. Silicone is the only thing Eternabond will not stick to. Eternabond products will remain flexible to minus 70 degrees F., which means it will remain stable in any climate; even under constant bombardment of UV rays in the heat of summer. Plus it is so easy to install. Just simply roll it out and press it down. Quite simple, no mess, very little prep, no clean up and Eternabond will stick to just about any type of RV roof including yours. Just be careful you have it where you want it. It is virtually impossible to remove.
Ask your local dealer about Eternabond products, or visit them on the web at www.eternabond.com. There are certainly other products available that will realize the same results, but I’ve had good success with Eternabond in the past.