Can I buy 6kg propane bottles in France

You can sign a contract, pay a deposit and buy a French cylinder - probably from a supermarket petrol station kiosk. If they want an address give the campsite address. The key thing is to get a cylinder which will fit your motorhome gas locker. Measure the space now -  you may find that a Le Cube cylinder will fit.

le Cube comes in two colours - red is propane, blue is butane . Both take a clip on fitting. If the supermarket petrol station doesn’t provide one you may have to buy one separately from a caravan / motorhome accessories store. You may even need rubber hose and jubilee clip to make up a DIY connection to an existing bulkhead regulator in your vehicle.

If I have got any of this wrong then others will correct me.

If your empty gas bottles are Calor you won't be able to buy a refill in France or Spain. There are many brands of gas available there, propane and butane, and you should easily find a size to fit your motorhome. Most Supermarket Fuel stations have a selection. Unless your MH is rather old you will have a bulkhead regulator suitable for use with Butane or propane which just needs an appropriate "pigtail" to connect to the bottle. Depending on the pigtail you have, it may be possible to just fit an adapter to the bottle end. If you use Gaslight cylinders your pigtail will already be suitable for some French cylinders, Le Cube for example. 

If you will be using EHU on sites you probably won't use much gas so you could just buy an adapter and a small Camping Gaz cylinder (eg 904). They are a very expensive way to buy gas but available readily in Europe and UK and can be easily carried as a backup.

As for your empty cylinders you will either need to take them to a Déchèterie or take them home with you. If you opt for a little Camping Gaz bottle your empties can probably stay in their locker.