check out the escapees forum and the hdt rally. Those are geared more towards class 8 trucks converted to tow big 5th wheel campers.
I just got done converting my mdt to tow a fifth wheel trailer. I put my fifth wheel right where it is supposed to be, just an inch or two ahead of the center line of the rear axle. Then built the box to be 5' ahead of the hitch center line to allow plenty of clearance to turn the truck 90 degrees to the trailer. You have to figure a 102" wide trailer is 51" from the center to the edge, then allowing a few extra inches for clearance. 23000# trailer tows great like that. Backing is far easier than with my dually as the turning radius of the mdt is far shorter, even with the much longer wheel base. I wouldn't consider moving the hitch back just for ease of backing, it can't help that much and if a guy can't back up with a standard fifth wheel location, he needs practice, not the hitch moved back.
Most of those trucks you see with the fifth wheel way back are done purposely to improve the weight distribution for ride and balance of a full class 8 semi that is has a way longer wheel base than your truck, and a much heavier unloaded front axle weight. Those guys have done careful calculations to figure out exactly where that hitch should be to get the front axle weight where they want it with the trailer hooked up and fully loaded. Just making an educated guess from looking at your rig, but you have a relatively short wheelbase, and the added sleeper is so far back that the extra weight is already more on the back axle than the front. I suspect your truck is already light on the front end and having the hitch behind the rear axle will only make that worse. My Kodiak has a longer wheel base than yours, approximately 12' from the cab to the axle, and with my 7' box built and the trailer hooked up (with the hitch just ahead of the axle) the front end was still light and rode really rough. When I stash about 1800# of cargo in the box the ride smooths out real nice. Which just confirms to me that the hitch has to go inline with the axle on an mdt like ours.
Of course on your truck it is not possible to put the hitch at the rear axle with as far back as your sleeper goes, you'd never be able to turn. So you are going to end up with the hitch at least 5' behind your sleeper, but my advice would be not an inch farther than you have to. No other choice. So the question becomes are you willing to stretch the wheelbase if the balance does not come out right? And you should try to do those calculations before it gets to that point. There are some threads on escapees that talk about that issue.
Here are some pics of my truck before I built the box, and under construction to give you an idea of the wheelbase and hitch location:
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