Carl, hope the below helps and is understandable.
I have completed the registration process for three campers now, but only for one from the UK, which was the most complex, the other two were for German sourced vans that were newer and had the single CoC document.
Can't help ref the older car and bike but I understand that it is relatively straightforward and more simple than that for the modern motorhome.
For the camper, ifregistered before 2004 (probably much later being a UKvan) you will need Certificates ofConformity for each element of the motorhome.
The reason I mention date is that the law was changed toallow motorhome manufacturers to now issue a single document that covers thewhole vehicle.
In your case you will need one from Auto Trail and one fromMercedes if the vehicle had an Alko chassis that would also be required.
The Mercedes one should be pretty straightforward, mostmanufacturers charge, Fiat charge €150 no idea what Merc will want.
I'm not sure how you will get on with Auto Trail, if theycannot supply the required documentation, you will need to make arrangementswith the DREAL local to your property for a Demande de Réception à Titre Isolé , which will basically be a one offinspection of the vehicle, you will probably also need to get a certificate ofsoundness for the gas system, normally via Veritas.
You will need allof the obvious requirements to be fulfilled i.e.
Proof of residence.
Registrationdocument from the country the vehicle is currently registered in.
Proof of ownershipof vehicle.
Insurance for thevehicle.
Proof ofroadworthiness, in reality this mean getting a Control Technique certificate,which is the French equivelant of an MOT.
A document calledQuitus fiscal which you get from your local Centre Des Impôts (the localtax office), simply states you don't owe any taxes or duties in France on thevehicle.
Demande de Certificat d'Immatriculation, which is the applicationdocument.
The documents from the DREAL.
When your dossier is complete you submit it to yourPrefecture along with the required monetary charge, they will issue you with atemporary document and you will receive the Carte Grise in the post a while later,usually within 10 days.
Some helpful websites, in French but having a French wife should mean no problems there.