If you plan to have indoor logding in any National Parks, make reservations now
(if you can get them). This will, of course, tie your schedule to fixed dates there, but it would be a shame to have to blow through Yellowstone quickly.
The TransAm by motel was done last summer by a couple. Their journal is a tremendous resource for anybody who wants to do the same. They name the motels they used, their prices and their contact information.http://bicyclelife.topicwise.com/doc/Yumadons1
There's also a book with a lot of motel resources listed. The information is 10 years old now, so I wouldn't trust it without double-checking. I do own this book, but I found it of limited use unless you need to motel references, so I don't really recommend it for campers.
Stephanie Ager Kirz, Bicycling the TransAm Trail: Virginia to Oregon/Washington, 2nd Edition, White Dog Press Ltd., 2003, ISBN 978-0974102719.
Here's my two pieces of advice for using motels. (1) Don't watch the TV, and (2) Spend as little time in your room as possible--mingling with the locals is one of the best parts of the trip and you don't want to isolate yourself from it.