hi thought id just mention a few things on this
did my van up as camper 8 years ago
before doing floor painted with hammerite
whatever materials you use to line vehicle make sure they can breathe and dry out.owing to all the gaps that get left in double skin sections condensation will form and any plastic or waterproof layers on insulation will trap it.
i found that as long as you waxoyl the underside of the van atferwards you can put as many screws through the floor as needed to hold stuff down but try not to screw into closed sections chasis rails ect.or pipes
the screws do go abit rusty after a while but its only cause they are in contact with the cold outside and warm interior allowing condensation to get them,but the waxoyled outer stops the van floor rusting.
mines been like done as mentioned and after 3 years as a camper and 5 lugging around mowers and large amounts of waste often wet the floor is the only part that hasnt rusted.
russ.