We did (Me+Wife+teenage daughter and son) Yellowstone in July 2015 (from Northern California) pulling a Keystone Sprinter 26BHS behind our Ford F250. It was the trip of a lifetime
We stayed a Grant Village (no hookups) and loved it. It's closer to the west entrance and Grand Tetons entrances. They do have tank cleanouts, and have great showers that are a short drive from the campsite. Anything 30' and under will fit in many spots there. We stayed the night before our Grant Village checkin at the West Yellowstone KOA , which allowed us to checkin at checkout time (11am) and get a better choice of spots (this was a strategy we picked up here on rv.net)
Keep in mind that EVERYTHING in Yellowstone is FAR APART, and parking is limited at most attractions (so a TT is a good option ;) ). I'd recommend the "Ring of Fire" bus tour and the Roosevelt Cookout. If you're planning on going this year, both may already be booked for the entire season.
+1 for Colter Bay Village RV Park in Grand Tetons. I was amazed at how many whiny reviews it got on tripadvisor, some people just like to complain. We thought it was great, and has already been pointed out: how can you beat full hookups at a National Park. I'd allow at least 2 days for Grand Tetons, it's almost as awesome as Yellowstone, and about an hour from Jackson Hole (worth a visit if you're into touristy stuff)