I can say from first hand experience, having driven both Jolly and Superjollys that unfortunately the Jolly is just a bit too slow with a heavy body and small engine. The Superjolly whilst not quick is much better to drive, you can even get to 60mph in one!
I have seen the Superjolly for sale at European Classic Cars, I don't think it has ever been a camper, there is no sign of anything ever fitted in the back, it is actually remarkably original and looks like it has had an easy life.
If it were mine (And no I don't need a project) I would cut the back off, put a back on the cab, and then get a company who specialise in car transporters (like this
https://jdbodies.co.uk/ ) to put a body on it. (They do it for £1500!!).
Judging by what the one that sold at the Duemila Mille Route auction made, it would even make economic sense.
....just what you need Simon