First, stop trying to guess and stop drilling holes. Get a simple stud finder from a Home Improvement store and figure out exactly where that backer board is located, (or not located).
Second, if there is no backer at all, you can still attach the television support. You'll need to prep a good looking piece of wood. Then use Molly Bolts to attach the board to the wall. The more you use, the better it will be. Before attaching the board, apply some good construction glue to the back of the board too, (like Liquid Nails), and then attach with the Molly Bolts. Give the glue a few hours to dry and then you can attach your television arm, or holder directly to this board.
Just for an FYI, in my 2 previous campers, I did exactly this same thing, although it was on an interior wall, not an outside wall. However, I used a flat bracket, not an arm, where I could lift the television completely off and lay it on the bed when traveling. We never traveled with that television hanging on the wall. And, we never had any issues with the wall doing it this way when traveling.
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