On Verizon's Coverage Map, most of the white space is in the Western mountains - and deserts of Nevada. Idaho has the largest white space - and the largest contiguous Wilderness areas (where RVs can't drive).
In Washington state you will get better coverage on I90 where it crosses the Cascades, than on US2 or US12 (or WA20 or 410).
Cell coverage, especially the new digital, is line of sight. Older analog systems (1G) used high power (and maybe different frequencies), and could cover larger areas with fewer towers. But carriers were allowed to abandon those in 2008.
