Hello Jef,
I have watched your recent posts with interest, because as the matter has rolled on, it has very clearly demonstrated that the using these meters is not a simple task and in the hands of a non expert, the results can be very misleading, a position that I have extolled on a number of occasions.
Now to you particular situation. As the customer you can say that you want a second opinion, and arrange to have your own "Expert" do a report. _ the problem is that unless you can find an expert that is accepted by all concerned, then the outcome may not be have the authority you want.
If you seriously believe that the workshop is not performing the tests correctly, then you should ask Trading Standards for advice.
As Damian suggests you could ask the caravan manufacturer to intervene, but that may depend on how you are trying to deal with the matter is it warranty (SoGA) or through the manufacturers guarantee.
Ultimately you could sue the seller under SoGA and let the matter be decided by the courts, but there is no guarantee of the outcome. Definitely seek professional legal advice if you consider this course of action.
Finally, I have to say I am concerned that you may have jeopardised your position by publicising it so extensively in the forum. I do acknowledge that you have not named the businesses involved but having put so much on the internet if it can be attributed to you then it may be seen as a constructive plan to discredit the dealer/manufacture.
I strongly suggest you decline to post further details until after the matter is resolved.