I usually store my water jugs amidships in my car. The reason I put them in the centre of the car is because water is probably the heaviest thing you'll be toting and it will balance the load. If you're pulling a trailer, do the same, for the same reasons: Put all your water jugs in the middle of the vehicle, whether it's towing or towed.
Pro-tip II: DO NOT put them in the roof rack. They're blocky, heavy, and could explode from all the sun beating down on them. Put your solar lanterns in the roof load, not the water. You don't want to get to the Playa and found you've lost one-third to half your water because the jugs popped. "Pop" means the seals around the lids. Putting water in the roof rack also makes you top-heavy and there's some nasty winds and winding curves on the way to the Playa: recipe for rollover.
Pro-tip III: Water is crucial. They won't let you in unless you have enough water for your group, even if you have a ticket. #1 priority, don't skimp on the water. Everything else can be worked around and toggled with. Trust me, you'll find something to do with it