Washington State allows motorhomes up to 46' max overall length. http://apps.leg.wa.gov/RCW/default.aspx?cite=46.44.030
"It is unlawful for any person to operate upon the public highways of this state any vehicle having an overall length, with or without load, in excess of forty feet. This restriction does not apply to (1) a municipal transit vehicle, (2) auto stage, private carrier bus, school bus, or motor home with an overall length not to exceed forty-six feet, (3) an articulated auto stage with an overall length not to exceed sixty-one feet, or (4) an auto recycling carrier up to forty-two feet in length manufactured prior to 2005."
You can tow a trailer as well but you have a maximum "combination " length of 75' including the tow vehicle. Semi trucks operate under a different rules because the "tractor " does not count towards the length I.e. you can tow a 5th wheel trailer up to 53'.