The Law
UK
In the UK vehicles being used for work purposes have to show stickers indicating the vehicle is carrying compressed gas. If the vehicle is not being used for work, i.e. a camper van, you do NOT have to show the stickers.
However, it is considered good practice to have the stickers if you are carrying compressed gas. The fire brigade find great benefit in seeing the sticker if there should be a road accident, as it helps them contain the incident more quickly. But, only show the sticker when you are carrying gas, otherwise the fire brigade will be miss lead.
Note: If you are showing the gas sticker on your vehicle, expect to get stopped occasionally by the police when they check how you gas is stored.
In the UK:
a vehicle is permitted to carry a maximum of 2 x 10 liter bottles of compressed gas, unless the vehicle is ventilated with a rotating rooftop device, then the limit is higher.
Flammable gas MUST be carried upright at all times.
Warning diamonds must also be displayed, if for reward. Green for High Pressure, Red for Flammable.
BS EN 1949:2002 Installation of LPG Systems – Specification for the installation of LPG systems for habitation purposes in leisure accommodation vehicles and in other road vehicles