Phil's solution adds a new dimension to the discussion.

To take that to a new level of discussion, I would offer there are some factors that need to be established.

Basically, do you want the extra battery to provide added capacity when using a winch or axillary lights?

Or, do you desire the extra battery to provide added capacity to run accessories while keeping a primary starting battery alive?

This provides two different install options.

In the case of needing the extra battery to provide added capacity when using a winch or axillary lights, a straight dual battery install with no isolation would be optimal.

In the case of needing the the extra battery to provide added capacity to run accessories while keeping a primary starting battery alive, an isolator or even a secondary system with one or two batteries would be optimal.

In the second case, a battery isolator two keep a primary starting circuit alive would be a good investment.

That comes down to what is your intended use? Carefully consider your intended needs and base your mods from there...

If the option is for #2, care needs to be given to the type of battery used to run accessories.

Basically, starting batteries need a specified Cold Cranking Amps as a base factor.

If you want a long duration current draw, you need to consider the reserve capacity factor of a battery. I don't have the details handy but Optimima has info available about this.

So, if providing power for campsite accessories is important, you need a battery with a "deep-cycle" capacity that will provide current draw capacity over time with a quick recovery cycle.

Such a battery does not make for a good starting battery.