Roof replacement

Interesting comment about the white foam "sponge". I recentally discovered that type of insulation will absorb moisture and can develop black mould. The styrofoam "SM" type will not.
I would have to agree that removal of the old roof would be my choice.
Good chance to look for water damage.

I tackled that job on my last RV(a Class C MH).
After peeling off the rubber membrane, I found some rot in a couple corners of the plywood. I cut out the 3/8" plywood around the bad areas and found rot in the 2x2 ceiling framing and water soaked beadboard (white foam insulation board). The beadboard was like a sponge. So I pulled up ALL the plywood and removed the "sponge" board. Without damaging the interior ceiling board, I cut out and replaced any rotten 2x2s. The 2x2 framing around the AC vent opening was weak and inadequate so I doubled up all the 2x2 ceiling joists and AC/vent openings with liquid nails and screws. Since everything was open from the topside, I added wiring for those extra little things like a MaxxFan, outlets, and speaker wire. Next I installed blue XPS foam board and glued and screwed 1/2" plywood to cover everything. There was no flex at all when walking on it when I was done. Next I filled the screw heads with plastic wood filler, installed the rubber EPDM roofing with the roof glue, cut out the openings and installed new molding trim using putty tape and stainless screws. Last, I installed new vents, a new AC, new tv antenna and drizzled the Dicor over all the topside screws and such. Took a few weeks for the entire project working inside my polebarn.
I wouldn't care to do it again but it really helped in the resale of it.

I replaced the roof on one of my slides on a previous trailer. It won't take but 1/2 hour or so to remove the old roof. Use a sharp knife and cut around everything and peel it off. You will want to inspect for any water damage to your roof decking anyway. You also will need to remove any fittings on the roof like, trim, skylights, vents etc. Almost easier to replace them rather than trying to clean them up and re-using them. I wouldn't leave the old roof on and go over it. There would be no guaranty the new one would would stick. When you found out it wouldn't it would be too late.

B.O.

Going over the existing roof shouldn’t be a problem IF…
The existing membrane is Still well attached with good adhesives…
The adhesive use to install the new membrane is compatible with both the old and the new membrane…
The new adhesive won’t cause the old rubber to swell or penetrate the rubber and cause the old adhesive to release its grip while curing or over time…
Prepping is everything almost, or at least the most important thing…