The most efficient way to pack all your stuff and not have wasted space (or run out of a very limited space) is to make a staging area.
To make a staging area, simply do this —
1.) Gather ALL the items you wish to take with you on your camping trip (or road trip, or day trip, etc.) and set them next to your vehicle.
2.) Open all the doors, trunk, etc.
3.) Simply eyeball your available space and imagine where the items will fit best.
4.) Load the vehicle.
Having a staging area next to your vehicle allows you to pack items in the best spaces possible, as opposed to packing them randomly and wasting space.