Another good one, but a little farther away from Keller, is Cedar Hill State Park. It's slightly southwest of Dallas, and easy to get to. There are 2 loops with full hook ups - rate $30/night. Reservations, if needed, have to be made with Texas Parks & Wildlife. If your trip is planned around the Labor Day weekend and you plan to stay here, call now - this park fills up on holiday weekends!
Also, on the opposite (west) side of Joe Pool Lake is Loyd Park. It is run by the city of Grand Prairie. Very nice park, but no full hookups. Just an FYI - they also charge $10/day for your vehicle as this is patrolled by the city police and the fee helps cover this cost.
And, one more in the DFW area is The Vineyards Campground and Cabins in Grapevine (closest to Keller of the 3). This is run by the City of Grapevine. The web site is www.vineyardscampground.com
. They have all the info (rates, photos, events, etc.) on their web site.
Hope the info for the 3 are helpful. Safe travels!