It was me, that mentioned soldered joints, as an example of one difference, from ordinary domestic gas intallations, and those for motorhomes.
A person qualified with "Gas Safety" accreditation, for domestic gas systems, might not be aware of such issues. They probably wouldn't even check pipe runs, underneath the van, as another example.
My main point is, most motorhomers, would not know about the specific qualifications required, and might accept any qualification, as being ok, which it certainly would NOT be, in this instance. There are plenty of people, out there, who are only interested in taking your money! Beware!