I rebuilt an aluminum roofed fifth wheel in the late 1980's. No roof decking there either, just stapled pine boards of less than 2X2 inch dimensions. I contacted a local RV repair firm and inquired about the use of decking, and was advised to not use decking unless I was replacing the entire roof with a TPO type product. The concern the repair facility had was rooted in their belief that all aluminum roofs will trap moisture from the living quarters between the underlayment and the aluminum, and then the aluminum would corrode through from the inside out. (This is the reason we no longer use aluminum siding and roofing on farm buildings, the stuff just turns to a white powder if moisture is trapped between it and anything on the inside.)
I very carefully lifted my aluminum roofing, replaced the damaged wood with kiln dried Douglas fir, packed new fiberglass matting back in the voids, and replaced the aluminum without underlayment. The fifth wheel is leak free still being used by another owner in a fishing camp all these years later, with just regular cleaning and the use of seam sealant on the roof.
If I were to do the same repair today, I would deck with knot free quarter inch plywood and one of the modern synthetic rubber type roof materials. Simply because those membranes are so much easier to work with and last decades if kept serviced.