Just because they're 18 doesn't mean the divorce decree doesn't apply anymore, unless you didn't think ahead and have a crappy decree that doesn't account for college. If you didn't factor college into the decree, go back to court for a modification.
If your children are in college and you are claiming them as dependents on your taxes, you and your ex should be responsible for their healthcare and education. With health insurance now able to cover adult children to age 26, make sure your modification addresses who will keep them on insurance if your kids don't have it through their employers after they graduate.
Basically right now if your decree didn't spell this out, your ex doesn't owe you anything. Get back to court, deal with your ex, and leave your kids out of this. Don't penalize them because you didn't plan ahead.
FWIW, we have custody of both my oldest son and step-daughter - both teenagers - and get no child support or help from either of their absentee biological parents. They cover none of their expenses and will contribute nothing to their education. It's especially frustrating because when my SD lived with her mother, we paid child support, medical & dental insurance, co-pays, glasses, haircuts, clothes, school supplies, etc. and she doesn't even call or visit, never mind pay us. I understand your frustration and that it feels unfair (it is unfair) but your kids don't need to be in the middle. Just be content in knowing that you're the better parent. Go to court to make his continued responsibilities a legal matter that can be enforced.