36 foot x 12 foot 6 inches is over 30 m² it is not a mobile home, it is a pre-fab, so like any other house, flat etc, comes under LABC, and not classed as as mobile.
"For the purposes of this section. the term "unit" is intended to mean a vehicle and/or mobile or transportable structure in which all or part of an electrical installation is contained, which is provided with a temporary supply by means of, for example, a plug and socket-outlet."
It clearly from pictures was not temporary it showed a full consumer unit for neighbours which had gone on fire, which was not plugged in but had a SWA connection.
This does not of course mean it is not down to some landlord to maintain, it may well be, and if it was in England it would need an EICR under the new rules if rented, OK in Wales so likely does not need an EICR but she has lived in it for over 20 years (decades) and she washed the structure, one assumes inside as clearly rain would regularly wash the outside, and water clearly got some where it should not have got to.
If I washed my house the MDF floors would disintegrate. I would wash my late mother wet room, but no other room in the house.