Straight from RV Safety link: It's legal in Oregon as long as:
- Fifth wheel must be equipped with passenger coupling device and shall have a 2-inch or larger kingpin.
- Fifth wheel must be equipped with safety glass and some means of approved electronic or electrical communication between driver and trailer.
It's illegal in the State of Washington.
Editor's Note: No matter what the laws say, I, personally, do not believe it is a safe practice to allow any living thing, other than a plant, to ride in any vehicle which is under tow.