The cheapest option to start with is to just use a EHU extension cable.
like this one,
Crusader V893 3676: Amazon.co.uk: Car & Motorbike
amzn.to
View attachment 78315
they plug into the EHU point,
then run it into the VAN,
you then have 240v socketrs to plkug into,
They must have an RDC and double pole breaker for saftey . . .
View attachment 78316
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heres an older style thatrs availible. (i have one of these but havent used it yet)
i got it from GoOutdoors.
Maypole MP3766 Mobile Mains Power Unit with USB : Amazon.co.uk: Business, Industry & Science
amzn.to
.
View attachment 78318