Unless something has changed recently, none of the campgrounds inside the park have electric except Fishing Bridge, but popups are not allowed there. When we went to Yellowstone we stayed at a small private campground in West Yellowstone, MT. West Yellowstone has restaurants and other things to do and it's about 25 miles from Madison Junction.
When we go back we're going to stay at Wapati Campground, a USFS campground on US 20 about half-way between Cody, WY and Fishing Bridge. The campground is right on the banks of the North Fork of the Shoshone River and there are several sites with electric and water.
https://goo.gl/maps/rQGusfp7rR4DmiGi7
At Grand Teton, we spent a couple nights in a FHU site at Colter Bay RV Park on Jackson Lake. We'd gladly stay there again.