What van is it?
OK.
If you have car derived van (Ford Transit Connect, Berlingo etc), you will see that the cargo floor in higher than the footwells. Most vans still have the rear passenger footwell and a plate covers them to level off the cargo floor. Any water leak will get into your footwells and you will have a damp smell before long.
Recommend that you seal the floor, fibreglass followed by a top coat, and include a wall around the front and sides (where the rear doors would go) to ensure that any spilt water is contained and rather finds its way out the back doors.
If you have a slightly larger van, ie., the old Citeron Distch, Citroen Relay etc., then they generally have a cargo floor that is lower than the floor in the driver's cabin and you won't have the same issue.