What do you plan on running off the inverter? I assume that you'd want the batteries to charge while the truck is running, and use the batteries to operate some electrical equipment when the truck is off.
I've done numerous installations like these for a fleet of cable installers. I used a battery isolator (the relay type, not the diode type). Basically it detects the charging voltage of the alternator when the engine is running and connects the secondary batteries to the main battery/alternator so they can charge.
When the engine is shut off, the relay detects the drop in voltage and disconnects the secondary batteries. Many of these relays also have a manual connect switch which would allow you to use the secondary batteries to help the main battery when starting your vehicle (in cold weather or if you left the lights on by mistake).
Here's one example:
http://www.colehersee.com/pdf/hot_feed/D-617_SmartBatteryIso.pdf
As mentioned, use deep cycle batteries (like marine or RV batteries). Regular batteries won't last long in this application. Buy all the batteries at the same time and make sure they are exactly the same. Mixing different sized batteries is not a good idea - they won't charge or discharge at the same rate and for sure one will fail sooner.
Also as mentioned, your alternator would never put out enough power to run the inverter when the vehicle is running (not at full load, anyway). This is why I asked about what you plan on running off the inverter.
Connect the batteries in parallel and use #2 wire, keeping the leads as short as possible (inverter should be mounted right next to the batteries). You could use something smaller (like 8-10 gauge) for the wire from the vehicle battery to the isolator to provide the charging voltage to the secondary batteries.