they dont "have to" per se (afaik), but neither do you "have to" have your boiler serviced, or a PIR done of your house
most service engineers for caravan/mhs, mobile or otherwise that I know of at least do some simple tests as part of the normal service, leak test, CO test, visual/operational, and have done for some time.
The PIR seems only to have come in recently for caravans/MHs, and IMO this is only a good thing, imagine what can come loose with all the travelling/rattling around.
not heard any any ACTUAL incidents, but have heard of PLENTY of near misses, both with gas, and electric, I presume that with a caravan being more enclosed that the smell of gas or an overheating C/U is noticable sooner, before anything tragic does happen.
More insurance companies are asking to see a PIR, not sure how many in the grand scheme, but it has come up a few times on a popular caravan/camping forum.
Its quite possible that insurers are asking for certs, a coupld of local social clubs near me have had to produce PAT and PIR certs this year, along with certs for flame retardency for furnishings.