Larger critters have more restrictions
We have traveled 200,000+ miles with Marvin the Road Dog in recent years. Things to keep in mind -- plenty of water, consistency of food, temperature control (if it is not comfortable for a human in the vehicle, it won't be for your friend). Campgrounds are generally accepting of all 4-legged creatures under 30 lbs. But a canine companion closer to 55 lbs(?) will present some challenges because of restrictions.
Be sure to look at our <a href = "http://www.roadtripamerica.com/read/pets.htm">Pets</a> page for some other ideas and resources.
M.