Scratch what I said earlier about the energy per battery, since it's closer to 1300Wh than 5000Wh.
Before you get any batteries you should probably measure the power draw of your car
like MetroMPG did assuming a high use scenario like having the high beams on, the heater fan on high, the defroster on, the radio on, the wipers on, and whatever else you think could likely be on.
If it's 500W, and you want to drive for five hours, then you'll need at least 2500 Wh (500W*5h), which would require 2 of the 115ah Kirkland batteries at least, depending on whether the Kirkland battery is rated for 5 hours or 20 hours. If Kirkland is listing the 20hr rate, then you'll only get about half ([url=http://www.csgnetwork.com/batterylifecalc.html]Here's a calculator for battery energy versus discharge rate), maybe less, of the ah rating if you're using the battery at a 5hr rate, so you would need at least four of those batteries for a 5h trip at 500W.
Since you want to be on the safe side, you would probably want 3 to 6 batteries, depending on whether or not the Kirkland ah rating is for 5h or 20h, at least for 5h of driving at 500W. If I were you I'd start out with an alternator kill switch, one Kirkland battery, your starter battery in the trunk, and the right wrenches to swap it back in, since that'll allow you to see how much mileage you gain and how far you can go on one Kirkland battery. If you run out of juice, just swap in your old battery. If you find that the mileage increase isn't worth it then you can just return the Kirkland battery, and if it isn't worth it you can get however many Kirkland batteries you want and wire them up in parallel along with a cord to plug a battery charger into.