If it has NON power rear swing out windows. Use a coat hanger to reach between the seal and glass and trip the latch open. Now use a LONG stick / whatever to reach in and open the rear slider. If it has POWER rear windows you can use a THIN piece of stiff metal to go between the glass and seal and pop the arm that rotates out of the motor. It comes out easily when pushed toward the front of the van. It will also push back in easily when in the correct spot. If it has power door locks you can use a coat hanger or the antenna off the van to reach through between the door and the weather stripping about 6" up from the side mirror. Then trip the lock.
Used to be a common thing when I was working for Gtech. We ran Caravans as company vehicles. Had some folks who could have had 5 spare keys and they would still get locked out......