Hello Geoff and Eddie
With regard to the sealing of the Thetford fridge to the habitation area of the van the correct information is supplied by the appliance manufacturer, not the installer.
The Gas Safety (Installation and use) regulations quite clearly state that all gas appliances must be installed according to the manufactures instructions, and the relevant codes of practice. This means that the installer must follow all the relevant instructions. If Thetford state that the product must have a room seal, then to fail to achieve it is a breach of the regulations.
Thetford will have spent a great deal of time and money to establish the correct way of installing their product such that it affords the maximum safety to the user and that the product works correctly. These instructions will have been ratified by an independent laboratory, for safety under a wide range of operating conditions, and to comply with a number European Standards, before the product would be accredited with meeting the criteria for CE marking which it must have for a gas appliance to be sold within the EU.
Eddie, yes both unburnt butane and propane gas are heavier than air, and whilst the hot fflue gasses are lighter than air, all these can be blown about by wind. Consequently if an appliance is designed to be room sealed, but the seal is compromised, then it is possible that a wind of sufficient strength and direction could carry unburnt gas or flue products back into the habital volume.
Based on the limited information given, It is my view that the installer has probably failed to complete the installation correctly, with potential issues relating safety.