best way to fix up a fire damaged caravan bathroom?

our caravan bathroom went on fire last summer in france - reason unknown. all of the walls and the door were scorched badly i.e. completely blackened. the wash basin and porta potti etc were also damaged beyond repair. we cannot afford to buy a new caravan and have to either repair this one ( the rest of it is fine bar a bit of smoke damage to the roof and surrounding walls but that is minimal. could anyone give some advice about the best way of repairing the walls i.e. reboarding etc ?? would be very grateful for any suggestions - many thanks