I have been following this topic and must say that it has been , shall we say, "enlightening"!!!
First of all I will only add my perspective as a caravan engineer doing a full service on a van.
The price I quote is inclusive of all the necessary items to actually do the service, so oils, cleaners, hub nuts.
There may be additional items if certain items are required such as a pigtail or gas hose as these are not always needed at every service, and things like regulators etc which are not normal wear and tear items, but the customer would be spoken to and advised of any of these and their authorisation given to carry out the work.
As far as the items within the caravan, if something is present then it needs checking and that includes the smoke alarm and Carbon Monoxide alarm(if fitted) and as a matter of safety I would ask the customers permission to change the batteries, which need doing at a minimum of annually.
To take one thing mentioned in an earlier post about change of tenancy regulations 1975,that is a load of rubbish and most certainly does not apply to a touring caravan, and to charge any money to test a non existent Carbon Dioxide alarm is fraud.
As far as smoke detectors go there are two types, Ionisation and Optical, both detect smoke and heat with the Optical type not being as sensitive to certain conditions, so better suited to a caravan as they tend not to give a false alarm when cooking.
The Ionisation type are susceptible to moisture getting on the circuit board and causing a false alarm.
Testing either of the types of alarms needs an input of the type of gas being tested, so smoke or Carbon Monoxide, the test button does not test the sampling unit within the detector, it simply tests the elecrtical continuity .
Whatever way an individual engineer has his pricing policy it must be clearly presented with the cost of each item clearly visible.
The way some engineers and workshops like to charge for items which must be used to complete a service in addition to the advertised "service" price is wrong in so many ways.
There is a learning lesson here, if you get charged for all kinds of "extras" which you have not been asked about then you know not to use that service establishment again.
When I do a service under normal circumstances I am more than happy for the owner to watch everything I am doing and ask any questions they may have.
With the current situation that is still doable but with social distancing being important, but even so the customer is always debriefed about the service at the end.