Chiang Mai, to Lampang, fill up at Shell station on No 11 leaving Lampang. Follow 11, fill up again at PTT after right turn when 101 becomes 11 again, start of new New carriageway, Den Chai area. Then pass Nakhon Thai, Dan Sai and fuel again at PTT, Phu Rhua.
Turn right bypassing Loei onto Wang Sappung and left to Udon. Several fuel stations on this stretch. Then just 56km North to Nong Khai. Total distance around 700km. All fast roads, mostly good surface, and very few damaged or potholed roads. Journey time for me between 8 and 9 hours including fuel and lunch stops, so can be done in 1 day. Loei is not such a bad place to stay if you wish to split it. While the roads do pass over some mountains, they are highways, not really steep and mostly easily negotiated bends, nothing like the Chiang Mai to Pai road.
Also apart from a couple of busy areas, such as around Lampang and Uttaradit, and after Wang Seppung, there is not much traffic. Altogether a pleasurable journey with some lovely countryside and views. If you do stop in Loei over night then you can take the river road (No 211) following the Mae Nam Khong river from Chiang Khan to Nong Khai, much slower than the Udon route, but beautiful; that road does have some surface damage, but is easily negotiated.
As you mentioned Nakhon Thai, I assumed you were not thinking of going from Uttaradit to Nam Pat, Na Heo and following the border road to Nong Khai. This does introduce some much smaller roads, some of which are in poorer condition, and would take much longer than the route above. Maybe better in the dryer season.