Some of the posts have highlighted a preference for blown hot air heating, and I think there are some good reasons for it in some caravans, but it isn't always the best so wet systems can be quite effective, more so in longer caravans.
Because the blown air systems use ducting with a substantial surface area, the duct its self will release heat in the bedding areas through which it passes. This can have the added benefit of ensuring water pipes and fittings in these space are kept above freezing when the caravan is in use.
It is also why underfloor ducting can be problem in some caravans. The standard Truma UR duct is not water poof so it should be protected with an outer sleeve at least. Whilst not is primary purpose the sleeve does provide some added thermal insulation, and there can often be a substantial improvement by adding some proper insulation around it.
One other trick that I have used in some caravans, is to make the hot air ducting create a complete loop or ring. This isn't always an option due to the caravan layout, but where it can done, it can make a big difference to the even distribution of heat throughout a caravan, especially at the farthest point from the heater.
Wet systems use small diameter pipes which loose far less heat (which is good for getting the heat around a longer system) but they don't warm the hidden spaces as much. This has led to some freezing issues in severe conditions.