I think there is some misunderstanding between carrying a gas cylinder and transporting cyclinders for sale. You can't tell me builders have floor ventilation and special cabinets built for their gas cylinders. What about motorhomes? How do you transport your barbecue cyclinder from the shop to home or indeed to your campsite in the south of france?
For motorhomes the rules are quite clear. The gas bottles are to be in a cabinet sealed to the inside, accessed from and vented to the outside. Caravans have always had their gas bottles mounted on the drawbar, once in the open but now in a purpose made enclosure that is still sealed from to the inside but incorporated into the 'lines' of the caravan. These enclosures are vented outside.
Tim,
From what I can gather the enclosure that you would need would seal the gas cylinder from the inside of the van and the vent hole in the floor would need to be in an area within this enclosure to release any gas that could escape through the hole to the atmosphere and not into your van. If you could use the hole for your hose as an outlet, then there would be no point in having the enclosure in the first place. Hence the reason why I would go for the 4 point Gasit tank that seals the connectors with a small enclosure on the tank with a vent pipe to the outside. They are also not that much more expensive than the refillable gas cylinder.
I don't know how the roofers get away with just sticking their gas bottle in the back, but then they take it out and use it outside, so they are just transporting it. When I renewed my van insurance last December one of the questions they asked was did I carry or use gas. They weren't interested in the diesel air heater I have.
Spruce