other than Araons suggestion dont bother with imobilisers, on a old bus they wont work get a engine lid lock, take a lead off or get a quick release battery clamp and lock the engine door
My mate used to have a Baja bug and he used to remove the rotor from the distributor whenever he parked up - only took a second to do and it never got nicked!!
My other bit of advice would be to remove/hide anything valuable inside your bus - it's an old bit of advice but makes sense!