Why not build an A/S motorhome on a long nose truck chassis - like the Kingsley Coach? Check out
www.kingsleycoach.com You could use widebody trailer sheetmetal, extend it down a little for basement storage, and even put slideouts on it, since you would have a strong chassis to mount then to. The best thing about it would be that the engine would be out in front under a hood where you could pull up a chair and get to it. You would also be using a proven and strong road going chassis and drivetrain. P-30's, after all, were designed to deliver bread, not to go on long over the road trips. And you would not be overloading the chassis like the 325 and 345's are. When you see a tag axle on a motorhome, you know that the mfg has exceeded the chassis design specs.
If someone has a couple hundred grand is loose change, we could get to gether and build a prototype...
Neal