My 2 cents worth owning a van a few years back, waterproof the entire floor section, and approximately 150 mm high on all sides.
It is very similar to waterproofing your shower base as doing a bathroom renovation.
Protection for a long time.
Remove the rear rubber seal and do the same, then put it back on. And do it similar to the rear door as well. Super tight seal and waterproof from salt rusting the van away.
