DW Mobile Home roof panel repair

[sandss115]

I have a double wide with a shingle roof. The roof has been leaking and has rotted the roof decking in a 12" x 12" area. The roof does not have rafters like a traditional roof but is a sandwich panel with the sheetrock forming the underside, insulation in the middle and then the roof decking forming the topside.
With traditional rafter roofing I would just cut from rafter to rafter, cut a patch piece, nail it to the rafters and then replace the roofing in that area. But with no rafters to nail to I'm curious how this would typically be done. I have my own ideas but just want to hear what others would do to fix this problem.
Again, I just need to put in a 12"-15" square patch and then replace the shingles in that area.

Thanks for the help.