After you find a suitable location for another battery, you need a way to have them connected while the engine is running, and separate when it is not. The closer the 2 batteries are to each other and to the alternator, and the gauge of the wires used will determine the charging performance to a point. The voltage regulator in engine computer is the weak link.
The easiest way of combining and separating the batteries is with a continuous duty solenoid sold at NAPA. When it sees 12 volts from your ignition key or other switched 12 volts source, it combines the batteries, and disconnects them with the engine off.
Everything you need to power you laptops and chargers must be wired directly to the second battery. The inverter you buy should be close to the second battery over fat cables. Use regular extension cords to extend AC power.
Laptops use more battery power than you imagine. Mine pulls 3.0 to 7.5 amps depending on the task. Lets say it draws 5 amps per hour on average. The average size group 27 battery contains 100 amp hours of battery. If you want your battery to have any kind of lifespan, you should not deplete it more than 50%.
So you really have 50 amp hours of usable battery, and 1 laptop can deplete this in 10 hours. Inverters are also only 90% efficient or so, so you just lost another hour. The DC to DC 12 volt car adapters are better.
Some electronics do not like the modified sine wave from a MSW inverter but this does not seem to affect laptops. Pure sine wave inverters produce nice clean power, for 300 to 500% more $$.
Also keep in mind the vehicle's charging system is designed to top off a slightly depleted starting battery, not charge a heavily depleted deep cycles or dual purpose batteries. To replace 50 amp hours into the battery would take 4 to 8 hours of highway driving to bring it up to full charge. Upgrading the alternator wont help much, because the voltage is controlled by the PCM, and it drops the voltage to low too quickly so as to not overcharge the single stock starting battery.
Read part one and 2 of this: