It may be deiseling. That is when they is so much carbon depsoits on the pistons that it heats up and acts like a glow plug and ignitets gas fumes and keeps running which pulls more gas in. Unusual for your mileage, a little early but if you use you van in stop and go traffic not a surprise.
I suggest some high quality fuel system cleaner that will remove carbon depsoits and sludge such as BG44K. You will have to go to a car dealership at their parts counter to find it but it is worth it. Nothing else works as well. It is $20 a bottle. I suggest two bottles each in a full tank of gas then run a third tank of gas and see how it does after that. Your oil may get very dirty after this so be prepared to change it.