That's what he says he thought at the time, and that's what makes sense, since the rocks are more interesting on the west side and it's just dirt road and sand (mostly) on the east.
He never mentions honking the horn (which is interesting), just shouting and waving his arms in the air. Sroads, here on this forum, went out there and says RV should have been visible all along the trail they were on.
My own theory was that if she came back toward RT after being unable to find the key to the RV (I'm guessing after 2 miles in 102-104F heat, with no wide brimmed hat, warm beer, and no water, she wanted shade and to cool off), she might have walked past where he was or gotten onto a spur trail. That could have happened when she left him, if they had both walked down to the dry creek bed. She might have remembered "path was straight" (which it was when they walked out, but now they had made little left turn and she might not have remembered that - people forget that kind of thing all the time).