How do you/we know that they aren't.
What we do know in here is there is a shortfall in parts available from FIAT. It's FIAT who are the villains of the piece, producing crap equipment as they have been for 10 years to my knowledge - remember Juddergate - remember the unreadable instrument panel - remember the cheap nasty leaking plastic oil filter housings, the list goes on and on.
I don't know for certain and I didn't say they wern't but it has not been suggested, although perhaps if overtime is being offered I would have thought the OP would know if that is the case and bearing in mind they have had direct dealings with the motorhome dealer, the motorhome manufacturer and the garage concernedI suspect they would have mentioned it. The OP said,
"So far, the combined might of Spinneys, Autotrail and ourselves has failed to elicit any response from Fiat other than “we have booked customers to deal with and they take priority"