A 20% reading is not that high, and I suspect the Lino prevented ventilation of the floor. The fact that the readings dropped soon after is good. But what concerns me is when this was done in July when you would have thought the van would be giving lower readings than in winter. I have no personal experience of Swift but many on here don’t rate them for this and other reasons. They do appear to have a poor reputation for dampness.
Are you buying from a dealership giving a warranty, or is there any residual water ingress warranty from Swift left.
Also what readings is the floor giving now, this could have been a spillage and only temporary.
I have a golden rule when making such a purchase, if you have any doubts don’t. But that’s just me.