Have you looked around on the escapees hdt forums?
HDT - Escapees Discussion Forum
I bet you could find the hitch height most common there. There's also the heavy haulers resource guide here:
The Heavy Haulers Resource Guide & HDT Rally Site - Escapees Discussion Forum
I've gone to two hdt rallys in Hutchinson, Kansas. Spending a couple weeks visiting with lots of experts in heavy 5th wheel trailers. My take on it is the axles/brakes/tires on these things are the most underbuilt and prone to problems areas. Knowing what little I do about a heavy 5th wheel trailer if I was to haul one I'd upgrade the axles/brakes/tires to the heaviest heaviest heaviest duty ones I could possibly get under there. No matter what amount of fab work is needed. I'd get some of those auxiliary axles from a dump truck with 17.5" tires, air ride, etc.... As far as I know nobody has put those under a 5th wheel trailer yet. But, there's got to be a way to do it. Those axles aren't that expensive and they've got good air brakes. And, if you did it right using the large air bags those things have you could lift two of them for manuevering into parking spots. That would be cool. At a minimum how about doing to measuring and figuring on what a finished height for something like that would be and mount your t/s hitch according to what that might be? Then a bit down the road you can upgrade those axles/brakes/tires and it'll be just right.
BlizzND, you could figure out how to add those axles couldn't you? The guy from Doonan Trucks was at the 2010 rally saying he thought it could be done. Somebody's got to be the first in anything.