What can you do about a awful rotten smell coming from the hatchback of your van especially when you drive with the windows down?

All I can suggest is try and find the source of the smell while the car is parked, remove bumpers/carpet/plastic trim/ whatever in the area of the worst smell to see if you can pinpoint the source.

Check under the van to see if there is some roadkill or similar dead animal lodged under there somewhere. If you find something under there, remove it (USE GLOVES & WRAP IT IN PLASTIC BEFORE TOSSING IT), then scrub the area it was stuck in thoroughly with a 50/50 water/bleach solution and rinse very well.

If it isn't due to that, you may have a water leak in the hatch that is allowing water to collect somewhere and go stagnant. If that is the case, remove the water & any soaked carpet/padding/etc. You will need to locate & fix the leak however or the problem will return.