A two stage thorough cleaning of the interior with household cleaner and then a dilute solution of washing soda should remove all the mould spores.
Then as you say, leave the fridge and freezer compartment doors open and make sure fresh air can enter and interchange with the air inside the van. Usually keeping any door or gas drop-out vents open will achieve this.
We've stored all of our Eriba Touring vans outside (under a breathable cover), and by following this process, have never had an issue with mould or mildew.
Robert