My take on this was to buy a CC, and do what you suggest but using a small fiberglass 5th wheel. Bigfoot used to make one and they can still be found used. Escape makes one currently. These are 20' trailers, but the box is only about 13', the rest is cabover. Remove the frame, mount it on a subframe that slides off of channels, just like loading a 40 yd dumpster. You could get by with the 178" wheelbase chassis, and have a pretty small overall package, demountable.
A new Earthroamer, even the small one, is around $500K. The bigger one $1M+. And they have a long order book. Now the build quality of the box is very high, but not that high.
Problem with a small Isuzu cabover or their competition, is they are very uncomfortable to drive for more than a half an hour.