We stayed at a number of KOA's on our trip this summer. We found them no more expensive than any other campground we stayed at along the way with the exception of Great Falls, MT. We did stop at one in MN that had no RV sites available but offered us a tent site for $20. We opted not to stay and found another campground about 20 minutes up the road. We had the choice whether to stay or not as I assume was the same choice the OP had.
We stay at KOA's or GS campgrounds when possible because they do seem to have at least a minimum standard to meet. Traveling in a class B we do use the bathroom facilities and prefer ones that are kept clean which I find true of most KOA's.