OK the update from a Dormobile member: According to mine, which I believe is standard, the gas bottles are stored opposite the battery in the engine bay. You can fit two 907 size bottles there, so you can keep a spare - there should be a retaining strap. The line goes through past the spare wheel well. I have a shut off valve between the spare wheel and the rear seat. Then the line goes down behind the rear seat and through the small cubby under the rear seat next to the sliding door. It then runs along the floor to the cupboard, and through the bulkhead to another shut off valve and then the stove.
Additional info from the same person: the line on mine runs through the inside of the van, underneath the flooring along one of the grooves of the base of the van. If I remember correctly, there are little metal tabs along the groove that help to hold the line in place along the floor.