Section 721 of the wiring regs (BS7671-2011) is dedicated to "Electrical installations in caravans and motor caravans" and states:
"A periodic inspection carried out preferably not less than once every three years and annualy if the motor/caravan is used frequently"
This should now be the new EICR (Electrical Installation Condition Report) not a PIR as shown in fig 721 of the BGB.
It should have also been supplied with an Electrical Installation Certificate when new.
Hope this helps