Just looked at the Daily Mail photo that you mentioned. You were right! It’s a travel trailer and doesn’t appear to have grid power. Might be a small photo-voltaic solar array or module in front of the trailer and on the left, though. Such small (or large) power setups can be bought as cheap kits or built larger and very nicely from better components. A power plant can even be built into a utility trailer (hobby of mine).
The area is hardly populated at all, and many people stay in travel trailers or RVs on lots for quiet getaways from cities, fishing, hunting, hiking, skiing at nearby resorts, writing books or whatever. ...many retirees. Real estate has been declining terribly for nearly a decade, so building houses in such an area doesn’t make sense for many (extreme and expensive planning and building regulations in that county due to so many migrating from northeastern states in past years to take local government positions).
Can a psycho mass murderer get into such an area? Well, yeah, just like anywhere else, but we’ve been accustomed to seeing them appear in the news as originating from wealthier suburbs with expensive houses (like Holmes, Lanza, the Columbine murderers,...most of them).
He may have power but does he have a computer? No info on recent online activity.