How strong is PMF?

So I’m looking at building an “Ultra light” that will pretty much be all Foam insulation board. ... my wife’s little suv can only tow 1200lbs ... So I’m wondering about the exterior Skin strength. Everyone saids Poormans Fiberglass is rigid, but does it give enough strength to stand up to 65mph highway speeds? Would I be better using actual glass and resign? Weight is top concern and I gather PMF is lighter, but is it strong enough?