I was planning in the not so near future to put a new roof on the ol'
doublewide. I was thinking of stripping the shingles, putting down an
inch or two of Styrofoam with steel roofing on top.

Dow tells me:
STYROFOAM (TM) Extruded Polystyrene Insulation has a maximum use
temperature of 165 F.
Please assure that adequate ventilation is created between the
insulation layer and the metal roofing so that the temperature will not
exceed 165 F.

Also I've had some ants that like to nest inside a foam insulated steel
door, leaving the insulation outside on the ground. They said: "Design
your new roof in such away that the insulation is encapsulated in metal
or treated lumber that will deter ants."

I don't like the idea of not being able to walk on the new roof. And I
have no idea how to ant proof an entire roof area. Ideas?