More insulation in Newmar Cypress?

Hey all, Newmar Cypress with aluminum studs, thin ply walls, and sparse fiberglass insulaltion. My son wants to add more insul, it's pretty cold/ drafty in winter, is closed-cell expanding foam a decent option? Drill holes, insert straw, and spray? Are there any accepted techiniques for this, or is it a dumb idea? Better to add 1/2" of polyiso foam board inside, and sacrifice a little interior volume? Any suggestions appreciated. Thanks!