It could be EGS (electro‐galvanized sheet) metal backer plate. They are normally put where cabinets are mounted, or on your roof where the ladder is attached, satellite dish, or Tv antenna mounting places. Not knowing your model layout not sure what is behind where your mounting your items. You could always give Thor a call and ask them. They would more than likely provide you with schematics(depending on the year).
2018 Four Winds 31Y