Go to the "Find and Reserve" at Marriott.com.

Don't put in a city, just select the state.

Click on "Find" and you'll get a map of the state with Marriott properties shown. You can hover your cursor over the icons and get more information as to type, location, and price.

Do that with the states along your route (and neighboring states, if you'll be traveling near a border) and plot your course that way.

It worked well for us during a trip to the Southwest last year.