Junk Man- I BELIEVE the Double Eagle sleepers are solid mounted to the frame, but connected to the cab w/ a flexible boot if the air-ride cab is still desired.
BTW- I am also looking into a rig like what you want, and you might want to check in with ICT, or Indiana Custom Trucks, makers of the Sundowner line of sleepers. I've been talking to their engineer, Tom Roose, and he told me that they now also will put their interiors in a cargo-type box for those not needing a full heavy-duty over-the-road-type sleeper, at a HUGE savings of dough. Check out their website and go to the photo section where a medium-duty truck with such a box, used as an RV, is shown. These folks are super-friendly and easy to talk to. The website is
www.indianacustomtrucks.com . Check it out and give Tom Roose a holler. He's a good guy and will help you in any way he can on your project without any pressure. I'm thinking along the same lines as you, trying to build a truck-RV without spending too much. I check this site all the time, so give ME a holler if you'd like to. Best of luck!