The isolation switch has ONLY the positive cable connected to it ,one end direct from battery via a 20a fuse to one terminal and the other end to the mover control box from the other terminal.
Under NO circumstances connect positive AND negative to the isolation switch as you WILL cause a direct short circuit which WILL cause a fire and destroy not only the wiring but battery as well AND set fire to the van.
The only wires that should be connected are negative direct from battery to mover control box and positivedirect from battery, through the isolation switch from the battery to the mover control box.