Hello Legs,
I quite agree that we don't know what a trailer MOT might entail, but for a number of reasons that I can think off and have already expounded, it is highly unlikely that the gas system in a caravan would be specifically included.
An MOT would only work if the trailer is separately registered in the same way that cars are. This would also raise the spectre of VED for trailers. It may also make trailer insurance compulsory.
Technically an MOT assessor would be more than capable of handling the testing of a caravans brakes lights and rolling gear and general chassis condition. It may require some minor retraining to deal with the overrun brake system, There might also be some need to add some specialist equipment to test the operation of the overrun, and some MOT stations may not be able to handle the height of some large trailers such as caravans.
An important factor is that caravans represent only a small proportion of the total number of trailers, and the Govt, would be hard pressed to justify making them a special case. There are legal difficulties with trying to make special cases out of general sets, how do you define what makes a caravan a caravan, because a soon as you set a particular criteria, someone then tries to bunk the system to evade the special 'caravan' only provisions.
MOT assessors have training to meet their current needs, but it is not sufficient to meet the requirements of competency to assess gas installations. This would need each MOT assessor to have additional training, which under current ACOPS is both long winded and expensive.
Now, I am not in favour of the Govt instigating an MOT, as it is another way in which they seek to control us, but I am in favour of improving general safety, which includes gas systems in caravans. It could be easily archived if all the insurance companies agreed to need evidence of an annual check to agreed an maintained standards on all aspects of the systems in caravans. If you like a private sector MOT or Caravan Annual Safety Certificate.