The model number indicates that it may be a 19' 1971 model. ?? I assume you have no title for the trailer.
Look for the VIN either on the front of the trailer, which what you probably found, or on the frame somewhere in the area of the hitch or the battery box. It could be stamped into the frame or a metal tag. On newer HiLo trailers the serial number is the last set of digits on the VIN.
Go here for the 1969 owner's manual. The first page mentions the location of the numbers. http://www.hilotrailerforum.com/f49/...on-voyage-836/