Google caravan scrappers or similar. There are two or three real big one about and it's not imposssible that you may find the interior fittings you need. You will have to tell them all the details of your shell, make model, age etc and you will still have the problem of collecting quite bulky items .
Years ago i bought a 15ft caravan and fitted it out as a mobile calibration facility for a chemical factory I was associated with at the time. This didn't ever leave the factory site and so insurance and conformity issues didn't arise. So an alternative might be to find someone wanting a mobile food outlet, workshop, studio, exhibition unit.
As others have said, a DIY rebuild will run into all sorts of problems, not least that you may find the specific cravan units eg show cubical, loo, washbasin etc. cost far more than their domstic equivalents - (which will generally be bigger and heavier)
Bare shells were available in the 1960s - usually glassfiber mouldings and some quite attractive, but the degree of fittings in those days was pretty basic compared to today. 2 ring gas hob, no shower, bucket type toilet.