About 1/2 way, the Wallyworld just east of the big road in Princeton, WV., exit 8, IIRC, might still allow overnights. Stayed there a couple of years back. A little noisy.
However, 12 miles north of Princeton @ Exit 20, is Camp Creek SP, about 2 miles off I-77. Mash Fork campground contains 26 trailer/tent sites, 19 sites with electrical hookup and 7 sites with water and electric hookups. Dump station. Open year-round with limited facilities from Nov. 1st thru Apr. 1st.
For FHUs, Fort Chiswell RV Campground is popular with the snowbirds. Located off I-77/81 at exit 80... 3/10 of a mile south.
As mentioned, there's a Walmart in Elkin, NC that I have shopped at, but not over-nighted. Allstays lists it as acceptable.