Not withstanding what the Prof has said. I think the legalities are arguable in line with unclear contractural terms and inferred contractural terms. But more importantly. Why does a dealership get itself into this position?......
To be totally accurate I haven't said anything - I wrote it.....😎
I fundamentally disagree that in this case there has been an unclear contractual term. The OP asked the business to carry out a service. The business had information published about the cost of a service, and crucially they did not state that parts and consumables were included. There is no ambiguity, if parts and consumables are not included they will be charged on top of the basic service charge.
Whilst I appreciate the work required to perform a service on a caravan will be largely similar for all models, there are a number of uncertainties about the detail of the work required. Things like the make of chassis, single or twin axle, dual 7 pin electrics or single 13pin. for the running gear, Then any items inside the caravan with multiple manufacturers, e.g. Wet or Blown hot air heating, Dometic/Electrolux/Thetford fridges, Different makes of cooker hobs, etc. It would therefore be difficult for a business to offer a single price for a service, without over inflating the price to cover model variations.
Generally where a company does include some parts and consumables, they state it clearly along with any limitations, otherwise customers could not be charged for new tyres or pigtails if they needed to be changed.
As consumers we have to be vigilant to watch out for advertisers little tricks. In the example I gave above there is a weasel word "from" meaning the could charge almost whatever they liked.
Contrary to Dusty's comment about dealers being very busy, In terms of sales yes, but most service businesses including caravan service operations have suffered through the Covid and many have had to make redundancies, or look for ways to cope with reduced income. Strategically the sale of spares and servicing items is usually a fairly lucrative affair, and for any business that might have previously absorbed the cost of service consumables into a service charge, the operating margins are now much tighter.
I'm not condoning what has been reported as happening, as it would seem that a number of less than satisfactory actions have taken place with the OP, but equally perhaps the OP's did not review the scope of the work included in the service cost before agreeing to it and his expectations were unrealistic.