Take some of the fuel out and leave it in a glass jar overnight.
Diesel is heavier than petrol and there should be a separation.
To drain the tank you will need to get the van off the ground and then disconnect a fuel line to syphon out the fuel. Don,t try the filling pipe it's anti syphon. The system will need a complete flush. And take off the filter or drain it. After you have the engine restarted with diesel change the filter as soon as you can as the petrol will be absorbed by the filter paper inside.
SAS
December 2005