I'll recommend Camping le Sologne at Salbris where we've stated several times. It's right in the middle of the village, well signposted, and you can get a simple meal there. There's a small bar/restaurant which is well patronised. The food is nothing special, but adequate for a quick overnight stop. The manager and his wife are both lovely people, and the pitches are fine. The toilet block is old fashioned but kept very clean, and you can walk into the village around the lake adjoining the site.
It's actually right on the N20, which runs parallel to the A71 at that point, and you can come off at junction 4, head into Salbris, and then turn north into the village. Go through the shops and you cross a bridge with a lake to the right, the campsite is a right turn, not long after the lake.