Add a battery to your car...
Mount an extra battrery in your car or truck... perhaps in the trunk, and then just take it out and use it when needed. A switch is needed to be sure you don't short out the leads when the extra battery is disconnected.
A '24' series is a nice size to carry and will run lights etc for a weekend camping/sail if your frugal. The '27' series battery is portable but you can get a strain or a hernia pretty quickly with one. Two '24' series deep cycle would be more juice and easier to move around than a '27' series.