Hi Halps and welcome to Eribafolk
I'm thinking that we are looking at the receiver post upright of the door frame here? My immediate thought is to look at the Eriba midrail that is fixed all the way around the van at waist level. That rail hides a joint in the aluminium exterior panelling and, if the sealant underneath the rail breaks down, it's a root cause of interior dampness.

As your water ingress appears to be very close to the door and with the constant opening and door slamming it is a place that the jointing will break down. Not an expert but my best guess.

Randa