Have never done it but in theory it should be OK.
The engine has it's own battery just as any vehicle does, so the van will start and run as normal. But the alternator in the Cali is also used to charge the leisure batts as well as the vehicle one when you drive.
I would pull out the relevant fuse from the fuse panel under the passenger seat, I think it is the biggest one, 80 amp without looking, before you disconnect the leisure batteries. This should isolate the charge circuit to them, so no short circuits from the loose cables as you drive about.
Steve from SMG might have alternative advice as he is the Cali technical expert on this forum. There might be some very good reason why you shouldn't do this that I haven't thought of.
Alan