if you don't need to book in advance

What we do when we're on a driving trip and don't know how far we'll get is to pick up a coupon magazine at a rest area. The coupons are listed by exit number or area and you can compare several that way pretty quickly. I browse through the book while dh is driving, estimate how far we'll get and look at exits near there. Find a cheap place and stop. There are often limitations like $5 more for more than x people, not good on holiday weekends, some aren't good on weekends at all. But we've never gotten to a place and had them not take the coupon. I wouldn't do it in a big tourist city though.