Put simply, no that's not an option with VM. They don't charge high users more, which is good, conversely there's no discount for low users.
You'd need a buy a 3G/4G LTE mobile wifi router (as a basic example) and a data only PAYG SIM card for whatever physical network offers good coverage at the caravan. A trip to Uswitch or other comparison site will show you the sort of PAYG data sim pricing.
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