You would die from carbon monoxide poisoning if you ran it inside your van. Instead, you can attach it to the back pretty easily and pass the power through the wall to maintain waterproofness. I don't know about the UK so I'll give you some US links for reference.
Trailer hitch cargo carrier: https://www.amazon.com/AA-Products-Inc-Anti-Rattle-Stabilizer/dp/B07ZKM33XN/
AC power passthrough: https://www.amazon.com/NOCO-GCP1-16-Inch-Integrated-Extension/dp/B009ANV81S/