Welcome to Truckconversion.net! You've figured it out. The MDT's are more expensive and less capable. Now, you are planning an "HDT," not exactly a "Truck Conversion." There is a very active group of people doing what you are planning, except they are pulling 5th wheel campers. They have a forum here: HDT - Escapees Discussion Forum
The first few sticky threads are chock full of great info for the HDT enthusiast.
The main forum there also has a rig for sale section that often has trucks already built and used for what you are planning. Most will have a 5th wheel hitch but a gooseneck could certainly be installed.
The other best source of info would be the HDT Rally
. It's a great gathering of HDT (and some Truck Conversions too) where you could glean a ton of info. You can often drive peoples trucks and see how you like them. And there's nothing better than chatting endlessly with people that have been doing what you're planning for years and know a lot.