Can you guys recommend a supplier for exterior wall sheeting that I assume is fiberglass with a foam core as used on RV's that don't use a metal exterior?

From what I know of this type of wall construction,... most RV manufacurers make their own or, outsource based on their spec's.
The foam core wall construction is set up in a model specific fixture & laminated under controlled temp & pressure conditions. I would doubt that you could get a supplier to do a "one-off" job.
I believe that FRP (fiberglass reinforced plywood) is available, probably somewhere in the Elkhart area. A good number of the RV manufacturers are located there and a long (RV) wall would be difficult to ship any distance.