If you want electric and water at minimum then I'd recommend Grizzly, or Rocky Mountain RV Park in Gardiner MT if you want to be at the north end of the park. We have stayed at both, and at Grizzly multiple times. Grizzly, near the west entrance, will make it a bit easier to reach the geyser basins and the central areas of the park, but staying in Gardiner will make it easier to access Mammoth Hot Springs and the wildlife there and in the Lamar Valley - this due to major road construction along the northwest section of the Grand Loop Rd.