Hello John,
Its not true to say that blown air is not the best system, What is the best is what provides the level of heating and distribution required by the the customer.
Now I know, because I have actually tested heating system in caravans, is that a properly adjusted and installed blown air system can be perfectly adequate with external temperatures down to -5C.
The system needs to be balanced out, which to the uninitiated may take some trial and error. and sometimes small modifications to the pipe runs can make a big difference. You can heat the air quite quickly, but it takes longer to geat the substance of the caravan, and that applies to any form of heating, so it will never be instant.
First though, the hottest air is going to be closest to the heater, it is also going to have the greatest flow, so keep the outlets closest to the heater only just open, and the further away open them a little more and so on as the distance increases. Often the there is a difference in the length of each leg of ducting, so use the diverter plate in the fan unit to bias more flow to the longer leg.
Arrange the vents to blow at the floor, because the coander effect means the moving air tries to stick to the surface against which its moving, and will travel further before letting go and convecting upwards.
Secondary work involves looking at the pipework. The longer a pipe is the more resistance to air movement it offers. Also the more bends there are will also slow the air movement down, In some caravans it is possible to re-route the pipework so it is shorter, or to remove some bends. In one particular caravan design, I was able to suggest a shorter route that removed nearly 1.5 meters of pipe. That saved the caravan manufacture money!
Tersary work involves looking at the junctions in the pipework. Most junctions are Tee's. If the flow into the fitting is across the head of the T, then the air flow preferentially tries to travel across the head and only a small proportion of the flow is diverted down the stick of the T. The situation is not always as clear cut as that as the degree of preference is also affected by the amount of ducting and the back pressure/ resistance to flow it provides. In some installations the fittings are not aligned to give maximum flow to the furthers points, and by re-fitting the T's to maximise flow to the far points can make a significant difference.
Another major improvement can be achieved by creating as complete ring of ducting around the caravan,any excess air in one leg can the supplement the air in the other.
All caravans have some floor level ventilation, and some times pipework passes over the vent, or is in the same restricted void as a vent(e.g. under a fridge). You must never cover up the ventilation holes, but you could lag the pipework so it looses less heat in these areas
Some lengths of hot air duct are deliberately pierced to provide warm air ventilation into bedding lockers, and up the back of false seat backs. If you feel too much hot air is lost in these areas you can always cover up some of the piercings with duct or masking tape.
As a rough indication of how effective some of these measures can be, One manufacturers caravan as delivered was exposed to -5C and the heating was turned on. In its as delivered state, we measured hot air at about 28C at ceiling height in the main saloon, and only 8C at roof level in the rear bathroom. Floor temperatures in the saloon varied from 2C to 7C depending on how close you were to a particular external vent. After tweaking the system, the saloon came down to about 21C at roof and 18C at floor level and the bathroom improved to about 17C at roof and 15 at floor level.
The bathroom could be made warmer at the expense of the saloon by reposition the flaps in the duct system.
To answer a comment you made in the OP - hot air systems are not necessarily inferior to other systems. its often down to the knowledge and experience on how to get the best out of them. But that applies to everything not just heaters.
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