You don't want to be converting 12 to 240v then having to use 12V packs to bring it back down again as the losses would be huge!
You just need to get some LED lighting, the best stuff we use in our works vans is strip lighting its low consumption and is bright as anything!
Something like these would be ideal
http://www.ebay.co.uk/itm/2x-48cm-48-Wh ... 3f1818fb06Oh Halogen lights use a lot of power and produce a lot of heat, I ended up taking mine out of my old camper as they were melting all the backs and wiring behind the lighting unit.