Your allowed to carry a certain weight/amount of gas in a vehicle such as a van or car, but I would be very surprised if
you would be allowed to run a gas appliance in the vehicle especially if unattended and in a built up area.
Unfortunately it one of those things where we will never know the truth until the worst happens, but I honestly believe its only a
matter of time before some Clampet with a cheap fleebay water heater answers all our questions/ concerns.