The channel is filled with water

When standing outside of van, pay attention to the habitation inner step leading edge that's immediately below door (when closed). There is a narrow gully 5-6 mm wide and today found to be brimming with water approx half cup full. Imagine it's rainwater that gets trapped with nowhere to go. Can't see that it causes any lasting damage other than perhaps turning into grunge over time.

This has happened to me and I wondered if I hadn't closed the door properly or if the wind and rain were extra strong. Like you I don't think it can cause damage as it's in the 'plastic' part of the lower door frame.

The problem it causes me is that I can't open the door when the water freezes. When this snow melts I'm going to see if it's possible to file a few cross channels to let the water escape.
Al.

Is it possible to drill a small drain hole in the channel to let the water escape? I found our inner step ( 2013 Broadway FB) filled with water from spray entering through the air vents in the sides of the inner step. I fitted a rubber flap over the outside vents but left room for ventilation. Never had a problem with a damp inner step after that

Yes, a small drain hole sufficient for water to escape but not allow spray from underneath to cause same water reservoir.