Another piece of evidence is that if you look at familsearch.org and search for a George Devol, Nancy had a brother named George Washington Devol, born in Tiverton in 1780 and died in the West Indies (presumably he was a mariner, as were many people in that area at the time) in 1821. He apparently lived in Bristol, RI.
If Ephraim and Nancy were married in RI, there would more likely than not be a civil record of it somewhere, my best guess would be in the Tiverton section of Arnold's "Vital Records of Rhode Island," vol. 4. The next time I'm in the library I'll take a look to see if there's a listing.