You think we would just happen upon these motels as we drive or worth booking in advance? Never done anything like this before
We've done a couple of extended Euro trips and we usually plan in advance. there are plenty of cheap hotels/motels on the out skirts of every Town and City but I just prefer to have it done before I go. A few hours on the computer with Google and you can plan a route and book. Most of the time booking is fully refundable too.
Go though Booking.com or Hotels.com and look at the chains Steve mentioned too. Formule 1 Hotels in France are my favourites for pikey stays!
You can find some interesting private accommodation on Booking.com and the like too. A few years ago we found a fabulous B and B in a village outside of Budapest for less than £20 for the night.