From the information that you have, my guess would be (assuming that the first digit in the type might be an I):
Thirty-One Foot Airstream Sovereign
International Land Yacht with twin beds produced in 1975, but sold as a 1976. If this is the case, it should be between 31 and 32 feet when measured from tip of tongue to edge of rear bumper, and the coach should have tandem axles.
So far as value is concerned, it would vary greatly depending upon the conditon of the major systems. This would include such things as the axles, frame (this coach would be of the size and era where rear end separation as well as frame sag could be issues), condition of the exterior skin, functionality of major systems (refrigerator, furnace, water heater, air conditioner, water tank, pump, holding tank(s), water supply lines, Univolt, electrical system), conditon of interior componets, and soundness of the floor system. A thorough inspection is necessary to ascertain a value, but the site linked below should help in determining a value based on such an investigation:
Vintage Airstream Price VS Conditon
Good luck with your investigation!