Signal Mountain, Colter and Gros Ventre campgrounds are all great and a very reasonable price. However, they don't accept reservations and not all sites have electric. Colter and Gros Ventre each have 300 sites and rarely fill. We've always gotten a site in all seasons with our 40' motorhome. Signal Mtn. has a 30' max.
There's also Colter Bay RV in the park and next to the above-mentioned Colter campground. The RV park has full hookups and if you need hookups that's the place to go. Flagg Ranch is 'rough' compared to Colter Bay RV. Victor, ID is too far to drive into the park daily, especially if you're a hiker. The Jackson RV parks are extremely expensive and you won't get the scenery that you would by staying in the park. There's also a RV park in Moran but I understand it's not good at all.