Need Your Screw Method Advice - 1968 Travel Queen Resto Mod

Given the two choices number 2 seems less risky . Trying to hit the center of the 3/4"plywood wall after drilling through 2+ inches of wood will be a challenge with multiple screws. I would also use continuous metal gusset rather than individual straps in something thicker than ordinary aluminum flashing .

A third method would have been to make the front and rear walls overlap the floor as the sides do and screw in from the side .