Hi
Trouble with James solution is that the outlets closest the inlet will get the majority of the supply, where the outlets furtherest away will probably get nothing. So the middle barrels will fill up and overflow while the end barrels will remain empty.
Fill the barrels first from your supply when you get home and then fill the van in the morning. If you use an IBC tank, that holds about 1000l so even if its not completely full, you should have enough to do the everything together.
Perhaps the money could be better spent on a solenoid cutoff valve so your tank will always be full.
Spruce