As John says, reserve your national park lodges ASAP. Commercial bus tours seem to suck up most of the rooms by the third week in January.
We normally did not plan further than a couple days ahead. From Virginia through Kansas there was almost always another town 1-2 hours away, and depending on heat, headwinds, and fatigue, we'd adjust our daily mileage. From central Colorado northwest through Washington the towns were spaced about a day's travel apart for us; if you want to stay in a motel, you stretch or shrink that day's riding to fit.
On our blended TransAm and Northern Tier route, Guffey, CO, Jeffrey City, WY, and Newhalem, WA were the only "towns" there were no motels. (If we'd had to, we could have stretched to Fairplay, CO, and Concrete, WA.) Weekends in the northwest were often tight, but we managed to find a place every night we needed to.
Hate to advertise for them, but I was amazed at Verizon's cell coverage. All across Kansas, and in almost every small town, we got a signal. If you need to call ahead for reservations, you'll find coverage. Often, though, we'd just split up and go to a couple of different motels and ask for room availability and prices. I think we saved more with that strategy than AAA!