Hello Dougie,
The addition of a properly(1) wired socket cannot void the warranty, as your warranty exists between you and the seller and only relates to the condition of the caravan at the point of sale.
It may however affect your manufacture guarantee.
The only way you can establish that is to ask the owner of guarantee, in this case the caravan manufacturer.
(1) A properly wired additional socket will take into account all the relevant regulations and wiring practices. If you are not fully conversant with these regulations you must refer the job to a professional.