North America has a lot of big question marks because we have a lot of very hot, dry areas. Every form that transformed creatures take appears to be more or less, well, gooey. If they can't handle desert or semi-desert life, then isolated outposts should exist all over western North America--wherever there's a spring and a lot of open ground (for lines of sight). They could exist in eastern Oregon, southern Idaho, southwestern Wyoming, southeastern California, pretty much all of Nevada and Utah, a bit of Colorado and New Mexico, and most of Arizona, not to mention a long swath on both sides of the Mexican border with Texas. These ecoregions are the Great Basin, Mojave Desert, Sonoran Desert, Chihuahuan Desert, and Colorado Plateau; they comprise what the movies call the Wild West. They cover an area about the size of Portugal, France, and Spain combined.
By Year 90, there could be established networks of little towns throughout the Wild West. In the old days, people always followed watercourses when they had a choice, but unless a particular area had been confirmed clean, the people of Year 90 would go through the high desert instead. They would probably use camels (there are camel tour companies here and there in the Wild West in 2016) and burros in preference to horses, because horses aren't as well adapted to the desert. If there were enough people in one area, they could get together and clear out a ranch or what have you that had been taken over by the transformed. (They would probably call them something like monsters, zombies, or Borgqueens.)
Oh, and: Las Vegas would lose most of its water when the electricity went out; it's either pumped up from deep wells or piped in. However, it was founded near a natural spring, which was restored before 2013. Las Vegas means "the water meadows," referring to the good grazing around the spring. The city has miles of storm drains meant to draw off flash floods. So imagine a team sent to salvage in 'Vegas, wandering among the silent casinos and dead neon, camels padding past the vast and fantastic buildings of the Strip, always wondering if somewhere beneath their feet a Borgqueen is sleeping, waiting for the rain.