From the Truma website
http://www.truma.com/uk/en/heating/combi-6.php
The technical specification for the gas Combi boiler show that it does require a constant 12Vdc supply when the appliance is switched on.
As there is no convection option, the device will use 12V power whenever heat is demanded.
It has three power consuming states
1. At rest (Turned on but no heat is demanded) 0.001A
2. When starting up (just when the thermostat clicks onto demand, and I assume its the inrush current to get motors running and gas valves opening)which lasts for only a few seconds 5.6A
3. When the appliance has started and has settled into its running mode 1.1A.
In mode 3 running it uses no more power than the Trumavent fans fitted to the traditional convector heaters.
By comparison, the Alde heaters also require a 12v supply as detailed in :-
http://www.alde.co.uk/downloads/alde_3010_instruct.pdf
The manual is not too specific but it implies that the heater only requires a maximum of 1A @12Vdc and no more under any circumstances. It does not provide standby power requirements
So by comparison, the Alde uses less 12V power than the Truma. but the difference over say a 24 hour period will be quite small overall.
Note: The 12V performance gives no guide to the efficacy of the heating within the caravan.