Google Maps suggests US2 east to US97, then north (with detour around Kelowa via Merrit). The 2nd fastest is US395 to the border, 3 and 33 to Vernon.
Halfway appears to be around Kelowna. If you like city store parking lots that's probably the best area to find a place to stay (or Kamloops). But I prefer Provincial Parks, with tent friendly camping spots and after dinner hiking.
I like Kettle Valley PP at the junction of 3 and 33. Haynes Pt in Osoyoos is a premium campground - close spaced sites, but most waterfront. There are also PP between Merritt and Kamloops, plus some cheap (and rough) rec-sites. Logan Lake has a nice town campground on a small lake. North of there is a large PP, Tunkwa Lake. The road doesn't have a prov number, but is decent paved.
BC Okanagan valley is much more settled than Washington's part. It's more like the busy parts of the Columbia River. The Merritt area is an upland with forest and grazing. Kamloops is in a dry valley. 97 climbs out of the dry river valleys around Clinton. The 100 mile area is forest and lakes.