Never found a definitive answer to this on the interweb, and not for the want of trying. In my own experience of living full time in a caravan the electric option for heating, at 15p per kw/hr here seems to be the cheapest. Gas, propane at around £35 for a 19kg bottle comes in about the same, maybe a little more. 13kg bottles, the biggest usually found on boats, cost more per kg, and smaller sizes much more. Wood or coal burning is not really viable in the caravan for fire safety reasons but it looks to me would probably cost about the same as the electric hook up so would likely be the way to go on a boat. There's lots on the internet about energy costs for houses but nothing much about the costs of energy for off grid heating/living.