Converting Single windows to Double Pane?

I have to assume you are talking about the frame around the actual glass and not the frame the glass units insert into. A double pane glazing unit will be thicker/wider and will not fit inside of the same frame the single pane unit is wrapped with. There is also the possibility that new double pane glazed glass assemblies will not fit into the same frame that inserts into the exterior walls of the camper.

You could accomplish a similar affect by placing plexiglass on the inside of the existing windows. I would want to cut the plexiglass to match the height and length of the existing window frames