Why is entrance floor damp?

 Most likely your door is not 100% tight around the edges,  Have you checked the door seal(s)?      They are quite easy to replace if necessary, and there is a place that sells all sorts of different seal profiles, so you should be able to find the correct profile(s).

Also check the sealing of door frame to body of van at the lower edges.

If the seal is damaged then try here:- https://sealsdirect.co.uk/

Colin

Agree it is probably the seal but have you checked that the drain in the bottom of the door fame is clear?