Jack,
The reasons for answers all over the place is there is NOT an industry standard for truck/trailer wiring. Many only come with running, brake and turn.
If you have a "battery to battery" positive and ground wired into the pig tail, sure you could make up a longer pigtail-- perhaps with just the positive and ground (since you do not need running, brake and turn lights).
But, the next question is do you have 120 VAC in your storage area-- where a converter/battery charger could do this.
And if you do a large gauge truck battery to trailer battery or 12 VDC distribution panel, be sure you have an in line fuse at both ends-- right at the battery, as either battery should be hot (assuming the trailer battery is installed).
The more details you give us on how yours is wired, the better we can answer.
