I just returned to the USA after living there for over three years, and watching a stage or two each year.
If you have an RV, hotels are not an issue. But even if you want a room every once in a while, there are many places not immediately adjacent to the stage route that should be available. You can get the detailed stage routes with timings from the TdF Web site in early 2007 (the general route is published there in October as stated previously).
If you get a few good Michelin maps, along with the detailed routes from the Web site, you can plan to be in the area of the stage almost any time without a problem, unless it is on a mountaintop finish, or over one of the famous bigger climbs. For those you need to get there a few days in advance, stake out your spot, and live there until after the stage is finished.
OTOH, we have taken back roads to the lower slopes of lMV, for example, a couple of hours before they were scheduled to pass by, and had great spots to watch from.
If you plan carefully, some stages allow you to watch twice. For example, at the start, and then by taking a shortcut to intercept them later in the stage. Just be sure that you know the start area, and park where you can gain fast access to the shortcut. Checking out the roads doesn't hurt either.
Enjoy the trip!