From a DMV search:
http://egov.oregon.gov/ODOT/DMV/vehicle/trailer.shtml
Recreational Trailers & Campers
Registering & Titling Trailers
You do not have to register a utility trailer, boat trailer, or horse trailer if it is less than 1,800 pounds "loaded weight" (the weight transmitted to the road, through an axle or set of axles when the vehicle is fully loaded). However, you may wish to title these trailers because most law enforcement agencies and insurance companies want to see proof of ownership if it is ever stolen.
* Application: Fill out the Application for Title and Registration of Manufactured Structures and Non-Motorized Vehicles.
* Registration fees: $81.00 for the first 10 feet and $6.75 for each additional full foot. If the trailer or camper does not already have a registration plate, a plate fee also is required (see registration fee page for plate fees). The camper and travel trailer fee chart will help you to determine the correct fee. Campers and travel trailers are registered for a two-year period.
One registration plate is issued and is required to be displayed on the rear of the travel trailer or camper.
Any application for title must include the title fee and any other applicable fees.