I've often thought a camper van would go very well with a sib and that I'd look into it when my boy gets a little older.
I've fitted transfers to a few campers, great big ones where the sides come out or little van conversions (I thought a LWB hi top sprinter was a common van to do this to). Some of the vans had been decked out to take motorcross bikes so had plenty of space in the rear (half height garage as the bed was above, still enough room to get a bike or boat under though). One I remember had a toilet and shower, was really well done.
I've often thought that if I was to look at a camper I'd look for something like this though;
Ford 6 Berth Motor home / Camper Van, with awning and sides. | eBay
plenty of room, out on the road no different to a car to drive (my Dad used to have luton backed vans and they were fine, though you're right about car parks) and has pretty much everything you'd need for a few nights etc.