There is a brochure in the library for a 1977. The 77 hi-lo, at 18' 11" says its weight is 2575 lbs and gvwr is 3300lbs. tires are listed as G78 x 14. Don't have any idea how that all compares to 75 18 footer. Someone may know- or you could try calling JR repair and maybe they would have an idea.
Rick