We had a 40' 5er with the same GVWR. During the 8 years we traveling that 5er neither of us felt like a king pin stabilizer was needed or would stabilize the 5er more. What helped the most was the Strongarm stabilizers on the front jacks.
If you really want a king pin stabilizer, used ones are quite often for sale, just check the for sale adds on RV discussion forums.