Hi. Well, that's a very common sort of query.
Most of your electric lights are now all 12 volt anyway.
All your water pumps are 12 volt.
Your fridge works very well on gas and your heater is gas.
What do you need 240 Volts for - TV? Water heater? Electric kettle?
The main difficulty with Hobby caravans is that they are not designed to run just off a battery - they are intended as mains hook-up. As a result their mains electricity distribution unit contains a transformer that supplies 12 volts but probably in two formats (i) 12 volts dc for water pumps and hot air fan; (ii) 12 to 14 volts ac which can only be used for lights.
The car connections provide relays that allow the 12 volts dc from the car to operate all the internals quite happily so you could (theoretically) connect your battery to the same connections BUT the caravan relays probably need a higher trip voltage - as from a car alternator - to make the change over.
Many Hobby owners in the UK have arranged (through dealers or through DIY) to fit an on-board leisure battery, properly connected through appropriate relays etc.
But you could just connect through a converter. There are more things electrical on modern vans of course, so watch total loading and recharge capabilities.
Volvo V70 (170bhp), traded in;
Hobby Prestige 540UF now SOLD:
Now (Oct 2010) Volvo XC90 tugs a Sprite Alpine 4