Brenda, to answer your questions:
1.Why,,because they are overwintering in a cosy, safe place.They will get in through any available entry point, including under the window rubers, door seals and of course the fixed ventilation.
2.How to solve it,,spray with fly killer on a regular basis, like each month, as they will have laid eggs, which will hatch, grow, lay more eggs.
Is there anything you can do about it,not really except maybe change storage location, but they will still overwinter in the van despite your best efforts to stop them