Salbris is also a good place for an overnight stop, just off the route described by John Douglas - Camping Le Sologne, which is just off the main road through Salbris, at the side of a small lake, with some lovely lakeside pitches, friendly management, sanitary blocks are clean but very dated - but everything works well and the showers were nice and hot. It does start to fill up from about 4.00pm - and there are several French families holidaying for longer periods there, so get an early pitch. There's a small bar on side, which does snack-type meals and you can walk to the shops and supermarket in Salbris (where there is also cheap fuel, so take the opportunity to fill up).
Otherwise, a bit further south on the A75 is the municipal at Issoire, or La Grange Fort at Parentignat just a bit south of Clermont Ferrand. If you get further south on the A75 there are lots of sites in Millau, some with superb views of the viaduct.
I've listed several alternative areas, because I don't know how far down you actually want to stay -- but Salbris and then Millau are easily do-able as your two stops - and then from Millau down to Spain gets you there in reasonable time. We may be doing a similar journey in early May.