Provided your unit charges the leisure battery from the mains then everything within your unit that needs 12V will continue to function indefinitely. If you remove the battery completely, you will not be able to operate anything that depends on 12V. This may include such things as the internal water pump, toilet, lighting, etc. - all depending on how your unit is arranged.
It depends on what the voltage requirements of you electrical equipment is. If it is all 250 volt then there is no point having a battery if you have not got an inverter to convert the 12 volts to 250. If it all runs off 12 volts then a battery might be needed to cope with surges in demand depending on what you run off 12 volts, Your 12 volt power supply will have to be capable of handling the total power demand (unless it is fused) in case of errors.