Hello Graeme and all,
There could be a significant difference between the type of unit your caravan dealer may be offering and the chargers from Halfords etc.
The units designed to be fitted in caravans have a dual role and thus are more correctly called power supplies. These differ from chargers in that they produce a smoothed output, and can be legitimately used to power some items of 12V equipment without the battery connected, and of course the are capabale of charging your battery aswell.
Battery chargers (as differentiated from power supplies) are designed to only charge batteries, and their output may not be smoothed. In fact for a charger without a battery in circuit its output can swing from 0V to 22V, 50 or 100 times a second depending on the type of rectifier used in the secondary of the transformer. This is like turning the power on and off 50 or 100 times a second.
Connect sensitive equipement such as a television to such a device, and firstly it may not work because of how 'lumpy' the voltage is and secondly the very high peak voltage may damage some components.
Battrey chargers must always be used with a battery connected. The internal impeadance of the battery dynamically tracks the the output from the charger, and it uses the excess voltage to dump current into the battery. This effectively smooths the output and makes the power useable.
Power supplies use additional components to control the output voltage, and to ensure it is smooth. For compliance with safety regulations, the output has to be controlled quite acuratley, and the components do do this add considerable expence to the product.
This could explain some of the cost differential, but haven't you noticed how 'special' appliances for caravan and boat always seem to cost more anyway?