I don't know exactly either, but there are some clues I thought of after I posted that story. I remember we were cheering for Flemish cyclist Fons De Wolf since he had won the stage the day before. I found out that he won stage 14 in the Tour De France of 1984 (which means my earlier estimation of mid to late 80's was not entirely correct), so the stage we observed from a distance must have been stage 15, from Domaine du Rouret in the Ardèche to Grenoble (https://en.wikipedia.org/wiki/1984_Tour_de_France#Route_and_... and https://en.wikipedia.org/wiki/1984_Tour_de_France,_Stage_12_...)
Sure enough, that passes through the Drôme departement where we were on holiday. I don't have the exact route for that stage, but it's described as a hilly stage so I assume it passed through the Vercors Massif on its way to Grenoble. That's most likely where we were that day, but where exactly I can't tell. I could ask my father but I don't think he'll remember either. It's quite a long time ago, after all.
There's an OpenTopoMap too, at least on Android. I think it's amazing how much information on the world around us we can now get with almost no effort and cost.
OpenStreetMap with the traditional topographic view. But I was not able to find where you wanted to hike to see the Tour de France.