How can I fix my leisure battery power?

I have a 2003 Swift Kon-Tiki on a fiat chasis. My son took his daughter for a weekend away but didn't realise how cold it was. Whilst the van was connected to mains electric he decided he needed more heat and switched on the engine for more warmth. Since then there hasn't been any reading on the controls from the leisure battery and the lights don't work from that battery either. I have done my best to check all the fuses (I may have missed one or two - its been so cold outside!) but haven't been able to find any blown. I have I missed a fuse or is there another problem? Thanks for any help.