Bruce,
I'm sure your roof looks fine. As a contractor I know better than to make a statement that EPDM roofing must only be installed over new decking. The substrate ONLY must be dry, clean and free of sharp edges, splinters, and any oil or grease. It does not have to be new. Putting a second layer of EPDM over a structurally sound primary layer is certainly acceptable but is usually not needed. This is usually only done in an area that has suffered severe damage, like a tree limb tear, etc. As I posted above, mostly all that is needed on any roof is to address ONLY the areas that fail on a regular basis. That is also where you will find any damage that needs to be addressed. In most cases, for about 30% of the cost, the customer can have a completely resealed roof and be good for another decade.
Since Firestone only certifies commercial contractors, any RV shop that is installing their EPDM product is using their own methods, which may or may not adhere to the manufacturers recommendations.