Let me start off by saying that I've honestly only seen one person with a fire bucket by the A frame. That was at a local site last year.
If anyones sees a fire they mustn't run, it's no good arriving out of breath and running around like an headless chicken. By walking it gives one a chance to assess the situation first and you arrive with a clear head. By assessing the situation you can come to a decision whether it's worth trying to put out the fire and not doing like Ian, running the other way. If anyone intends throwing water about they must disconnect the EHU first or delegate someone else to do it, also turn off the gas tap too if possible.
The owners might be away from the caravan, maybe at reception, and have dogs in the caravan. I can't speak for others but it wouldn't be possible for me to walk away in that instance and let it burn thinking "Oh well, they will have insurance". There are many single elderly caravanners and they may have suffered, for example, a C.V.A. They might have been having a smoke when falling unconcious, who will help them if everyone runs away and says its only a box. I suppose it will save on cremation fees later.
Like you Ian, I have seen burned bodies, two of them being firemen, but that doesn't absolve me of compassion to others, whether it be fellow campers or their pets.