Hello legsmanic.
What you say is not true. Touring Caravans are specifically identified within the Gas Safety Installation & Use Regulations, and thus the regulations do set out the statutory instruments covering gas installations within in them.
What they effectively say is that persons working on gas installations in touring caravans do not have to be a 'registered' fitter. It does not say that the other regulations do not apply. - They do in all respects and that is often forgotten.
The fitter must still be competent. All work must still be carried out in accordance with the relevant codes of practice for workmanship and safety. The relevant tests must be carried out to correctly to confirm the safe operation of the installation.
In all aspects of work (not just gas fitting) Health and Safety legislation supersede all codes or practice, and there is a duty to ensure that no person shall undertake work on a gas system unless competent to do so. All work must be carried out in a safe and competent manner. Non-compliance with H&S regs, is a dealt with through the criminal courts. Sanctions can include fines, prohibitions, and imprisonment. What is not widely appreciated is that this also applies to private individuals just as much as organisations.
The difficulty for the private individual is, how can they be demonstrably competent, without undergoing some assessment. Even a registered natural gas fitter may not have all the relevant skills and knowledge to deal with LPG and vice-versa.