Actually, it sounds like you're wanting to do a "Truck Conversion." Not an HDT. A T/C is a fully self contained single unit camper. Truck on the front end, camper/living quarters built onto the frame. There are a handful of companies doing Truck Conversions. Haulmark, Renegade, Showhauler, and Hawk are probably the most prevalent. Google them and go to their web sites. I think they will all convert your tractor to a T/C for you. And, I think they will also just build whatever you want them to. Sounds like you're thinking about having the box built and then finishing it off? Sounds like a great way to save some money and get it built just the way you want. Hahahahah, big huge "NO" to saving money. It will cost you much more to finish it yourself, and take way longer. Usually. The most cost effective way to get into the driver's seat of a T/C is to buy one for pennies on the dollar off www.racingjunk.com
, or some other place. Believe me. There is a forum devoted to T/C's, www.truckconversion.net
. Plenty have built their own, and several are building their own now. Most will tell you it's not to save money.
As far as the camper goes. Big huge "YES" for T/C's. They are a wonderful chassis for a camper. I had a Allegro gas class A before I bought my T/C. I don't foresee ever going back to a traditional A. Once you drive a T/C, that's all it takes. I know of one person that had a T/C, went back to a D/P (diesel pusher), and then went back to a T/C. Most of the T/C people I've talked to said they don't think they'd ever go to another chassis.
Sorry if I misunderstood your original post and you're actually looking to convert your truck to just pull a large 5th wheel camper trailer. That's usually what it means when you say HDT in the camper circles. Now, that's also a really good way to go, if that's what you want. Similar but different ball game than a T/C though.