As far as I know, none of these places has been canonically pinpointed on a map (except the named pre-war parks in Utah), so you could only sort of handwave in their general direction, at best. Ground Zero on the Long 15 is, of course, going to fall somewhere on Interstate 15 between Califonia and Vegas, which should be easy enough to ballpark with Google. Dry Wells is, according to The Vault wiki, south along the Colorado river, in Arizona, which would makes sense considering the direction of the Legion's push and the location of the boat you take to reach it. Big Mountain and the Sierra Madre purposely had their locations left unspecified.
You can safely bet that Navarro isn't anywhere in or near The Divide, though. The Divide was a pass through the mountains between California and Nevada, and Navarro is far to the north of there, not to mention being on literally the opposite edge of California, on the coast. Maybe your brother was confused by Ulysses saying the package that woke up the Divide came from Navarro. He also says that Navaro is far, far to the west during that same dialogue, though.
With some in-game sleuthing you could probably go pretty far towards picking out the stretch of highway that comprises the whole of Lonesome Road. We know it's one of two major arteries between central California and the Mojave in Nevada. I'd do it myself, if my computer hadn't blown up. There's even a real Ashton near the CA/NV border, though whether it's the one depicted in the game is uncertain. In real life it's a ghost town, so they could have just borrowed the name as a nod.
