In England the Smoke-free (Private Vehicles) Regulations 2015 make it illegal for the driver to smoke, or allow anyone to smoke, in an enclosed private vehicle carrying someone under 18, including motor caravans.
However the Regulations also provide the exemption that they do not apply to—
(a) a caravan or motor caravan that is stationary and not on a road; or
(b) a caravan or motor caravan that is stationary, is on a road and is being used as living accommodation.
“caravan” means a trailer which is designed for road use and provides mobile living accommodation.
“motor caravan” means a motor vehicle which is constructed or adapted for the carriage of passengers and their effects and which contains, as permanently installed equipment, the facilities which are reasonably necessary for enabling the vehicle to provide mobile living accommodation for its users.
“road” has the same meaning as in section 192(1) of the Road Traffic Act 1988(2).”
So, the law making it illegal to smoke in a vehicle carrying someone under 18 does not apply if: Your motorhome is stationary and not on a road, or is on a road and is being used for living accommodation.
The legislation for England is here: www.legislation.gov.uk/uksi/2015/286/regulation/2/made similar provisions and exemptions apply in Wales and Scotland. In Northern Ireland the Department of Health is in the process of drafting the regulations, which it is envisaged at this stage, will contain an exemption allowing smoking in caravans and motor homes when the vehicle is stationary and is being used as living accommodation.