Heading off road can be tough on all aspects of your vehicle, even the battery.
Vibrations and impacts from rough roads (or no roads), high temperatures under the bonnet and some accessories can impact a battery's life and performance. Therefore, it is important to choose a battery that is up to the job. In some cases you may have a dual battery system. This is for vehicles that have lots of power hungry accessories, such as fridges. This is where a secondary deep cycle battery can be utilised. The dual cycle runs the accessories and helps preserve the all important starting capacity of the starting battery.
As a side note, some manufacturers recommend that deep cycle batteries should not be allowed to drop below 50% capacity before they are recharged.