Are you replacing the roof because the rubber is bad, or you have soft spots in the wood below? I had to replace the wood deck on a trailer. After taking off all roof items and edge trim, I cut rubber at seams and took roof boards off with rubber attached, rather then trying to remove rubber. Then is used the removed panel for a template to cut new sheet. Did one section at a time so that I could work from adjacent surface to install new panel.
If you install new wood over existing roof, remember that edge trim will now be higher up, and you will need to make sure that trim will cover any gap between siding and relocated trim.
As I recall trailer was a 22 ft, and I believe that one gallon of glue is all that I needed. What a gallon will cover will be listed on can.