Are you doing the work yourself, or paying someone to do it? If you are paying for a roof replacement, I would STRONGLY encourage you to research some of the spray on roof materials out there. I believe Rhino Liner does a spray on RV roof, but there is another company that specifically does RV roof sprays.
Search this forum for a member named edatlanta. I believe he used a product called rvroof.com and absolutely loves the result. He paid decent money for the product, but found it to be a substantial upgrade over the factory rubber roof.