No wood!
Your really asking for trouble using a wood frame. It wont be long before you get racking that will lead to leaking where ever the metal skin or fiberglass if you went that way where constently moving.
Even traditional travel trailer mfg'ers who use to do this, dropped the practice.
The stronger you can make that external structure the less chance you'll have for movement down the road.
Bill
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2012 Showhauler 28'6" Motorhome on a Columbia w/ 450 Mercedes.
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