The value on an RV trailer that old is going to be based solely on condition. There won't been any book value to go on even if you could figure out what it is. The title may have the weight listed on it (depends on the state). If it doesn't, hook it up and tow it to a scale. This much is going to be true, it is going to be heavy, especially for a bumper pull. The odds are you aren't going to be going very far, so pick a slow traffic day, be sure you have the biggest truck you can get your hands on, and drive away. I would double check the tires, because if it has been sitting for years on the original tires, they might all decide travel day is the day to die. If that happens you are going to have a serious mess on the side of the road.