Welcome to the forum. The action is a little slow paced here, but a lot of guys with good info for your build. I did a little brother to your plan to tow my race car trailer, then 5th wheel camper. The GMC Topkick below. As to your build, have you considered eliminating the sleeper and sliding the box forward to mate to the cab and just cut less off the box? That would give you far more interior room to work with than making the sleeper into a crew cab, and even more for seats, couches, whatever. Think toterhome style build.
Opinions vary on hitch placement. I built mine with the fifth wheel hitch centered over the axle, which to my old school driving habits is the best handling and just what I am used to for backing up. Although I do also have a gooseneck hitch behind the fifth wheel, maybe 3' behind the axle, that I have used on occasion and that pulls just fine as well. I am in the minority, most guys converting hdt's for rv use put the hitch well behind the axle. Some say it is easier to backup, more like a bumper pull trailer, others do that just so they can make the wheelbase shorter for more maneuverability and still have a long box by pushing the hitch to the back of the frame. The one photo below of the box truck is the most extreme example I've ever seen, and I don't think I'd personally go that far. The Volvo is more typical of the rv guys, hitch 4' or so back from the axle where it is easy to drop the hitch down between the rails and give more room for a longer box or vehicle on the deck.
So I guess it's a trade off, the further up you put your hitch, the more you have to cut off that box.
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