If they were dependent on drugs or (especially) alcohol they would have looked a whole lot rougher coming out. Can't see them staying hidden that long unless they were hauling around their entire supply. They're also not exactly great at tidying up after themselves. I think if they did have a significant problem, it would have leaked somewhere so I don't exactly think that's what it is.
It just feels like there is a busload of missing reasons/context to the last decade of their life. The pieces we have just don't fit together to me. Sure, mental health could explain a lot, but they functioned well enough to dodge 3 police forces going balls out for like 2 months.
I'm not buying that they were so unwell that they were unable to manage basic stability on their middle-class maintenance money. So all those weird episodes living in tents, broken campers, general squalor was a choice, and I really want to hear the logic, even if it's crazy/culty logic.