Bear in mind that Voni and I usually leave home in May and return in September. In between may be several different "trips." To plan a trip the first thing I do is plug in my origin and destination to mapping software. I used to use Street Atlas USA but now mostly use Google Maps. That gives me a distance. Then I massage the route to roads and highways I would actually use, avoiding Interstates and major cities as much as possible. Then I plug in any locations I want to go to not on that route. That gives me a new, longer distance. I divide that distance by the number of miles I wish to travel per day, on average.
This distance will vary by locale as well as time of the year. Trips in the early spring and late fall call for shorter days so we don't start well before it warms up for the day. In extreme summer heat, say in the desert southwest miles per day get decreased for early stops to avoid late afternoon heat. If in the congested east days are planned to be shorter than in the west with wide open spaces and 75 mph (or higher) speed limits.
Now I have a number for days for the trip. I always add one or two days to account for bad weather or other delays. Then I start to actually look for destinations. If camping I identify campground possibilities. For potential motel nights I identify towns the approximate right distance.
I do not use Routes in my GPS. I enter waypoints as destinations and via points. I do actual roadway selections from a paper map in the pocket on my tankbag, along with the routing to the next waypoint from my GPS. I pick the roads. The GPS does not.
If planning to stay in a motel we check prices on-line. Sometimes towns 20 or 30 miles apart will have motel prices that differ by $50 or more. I'll ride an extra 30 miles or stop short too, to save $50 or more on the room price. Sometimes we book early. Usually about noon the day of the stay.
We have been known to book a room over the internet while sitting in a restaurant next door to the motel - or even from the lobby of that very same motel after the desk clerk said he couldn't give the internet rate. He did offer the use of the computer in the lobby.
Sometimes the plan falls apart. Severe thunderstorms, tornado warnings, localized flooding, freak snow storms, flat tires, bike problems, Montezuma's Revenge, and/or other things pop up from time to time. Remember those extra days built in to the plan? They are there for a reason.