Take the watermelon out now?
Try to do a carpet shampoo or some sort of deep cleaning of the area Mr. Watermelon was, then get some Febreeze and spray the daylights out of the whole van - all seats, THE "CEILING" of the van, the floors, any where there is fabric. Leave the windows cracked, and let it "dry" overnight. (Do all this right after you are done driving it for the day, so you don't sit on wet Febreezy seats.)
I didn't have a stinky watermelon, but when I quit smoking, I did the Febreeze thing. Cigarette smoke is horrible to get out of things, and that beat it very well. I used the regular Febreeze, and it just smelled...normal. No heavy scents or anything. You want to do the whole vehicle because I'd think by now, the smell would be in the fabric and foam, and even if you take the culprit out, you'd still have days of the smell coming back out of the fabric and foam. Yucky
Spray so every thing is damp, but not soaked.
I've heard of sitting a cup of vinegar or such in the car, but I've not tried it.