The following states allow riding in travel trailers. (Source: Woodalls State Polls ~2011
2011 Driving Laws for the US and Canada)
AZ, Indiana, Iowa, Kansas, Michigan, Minnesota, Mississippi, Missouri, Nebraska.
9 out of 50. Again, MOST states do not allow it, and some of the ones that do have restrictions.
Know before you go, check the state laws where you are going. As said, what is legal where you are licensed may not be elsewhere.