Check the domestic widebody thread. Other than designated D1 domestic routes such as between JFK and LAX/SFO, there are no permanent domestic routes offering lie-flat across the board for the entirety of the specific route schedule. Service will default to domestic F even if the aircraft may be an A359, as an example.

Here are some recent months of lie-flat flying examples, between:

ATL and LAS/LAX
DTW and ATL/SEA
JFK and ATL/LAS
All subject to change from one month to the next.