No Steve....not sure in fact I'd say I was wrong and the shunt WOULD see the load.
I remember where my doubt about using the chassis for a return path whilst using a shunt came from now and it wasn't a concern about the shunt 'seeing' the load because it will providing the leisure battery is ONLY connected to the shunt (as it should be) but the accuracy and reliability of the shunt while it's sharing a conductor (the shell of the van) with everything that's being powered by the starter battery electrics Inc starter motor, wipers, lights etc etc.
Discuss.