Guess you did not get an answer on RVnet.
Well, I only can assume 10 to 15% of tongue weight? 4,000 to 6,000 lbs?
Most likely the real answer is just load your real axle to the limit. 20K or 23K, depending on what number you believe.
Check out this one for the 3 prong.
http://www.motorcycle-carriers.net/m...rrier_c5a.html