One explanation for RT's words on camera regarding being a suspect would be that he felt/feels very guilty. Many husbands would. Many would have a hard time admitting they had lost their (vulnerable) wife in the desert. Maybe she went to sit down nearby, maybe they were way further out in the desert, but both are hard for him to admit (she was feeling a bit tired or unwell, in 100-104F heat, he put taking a picture above staying at her side, etc). People do rewrite things in their heads in order to feel better, that's for sure. And some people have very hard time admitting they were wrong as the real story would still be in their heads. This would also explain the polygraph issues. I think we all know people like this, they're not uncommon (can't admit to any sort of error on their own part, it's always the other person's fault).
If he lost sight of her further out in the desert, looked (panicked) for an hour, then went back to the RV and she wasn't there either, that's an entirely different scenario, would have been an entirely different search.