We also stayed at the KOA as it seemed the best in the area, though not the most convenient.
But a word of caution. The oil field boom is apparently making it difficult to find any opening at any RV park in that area. I would begin checking availability far in advance of planning to arrive there.
We did the tour on our own starting at the natural entrance. Plan on the elevator back to the top unless you are in good physical condition as the trip down from the natural entrance is long and steep. There are guided tours that take you to areas not available on your own, but we did not try any of those.