Just finished about 2 weeks in France [mostly in the Loire valley] with my mother, who turned 87 on the trip. She can get around but moves slowly and doesn't like climbing lots of steps. We brought the blue handicapped hangtag that she uses at home, and to my great surprise we found handicapped parking spots at many of the chateaux we visited and were able to park there with our hangtag. We confirmed that this was OK at Fontevraud Abbey (our first Loire stop); one of the cashiers was English and told us her husband uses a similar hangtag. You still have to pay if you are in a paid spot, but the hangtag was valid.
In addition, a number of the chateaux gave Mom [and sometimes me, her assistant] free entry because of her handicapped status. Sometimes they asked to see her card, and I said it was in the car; finally I thought of photographing the hangtag on my smartphone, and that was seemed to be enough. I think we had to pay full price at Blois, but everywhere else we asked they gave us at least one free entry.
I'll post a longer trip report when I get organized, but wanted to get this information out on the Forum as soon as I could. Overall, it was a great trip!