I mounted my Cobra 2575 in the passenger side box under the bunk in my Freghtliner Century. It is mounted vertically on the small divider just in front of the bunk HVAC unit. There is always airflow there. Even when the bunk fan is in the "off" position, the fan is actually running on low. It's also convenient there. I can easily lift the bunk and turn it on and off when I need to. I run a couple of short extension cords right through the pre-existing vent holes.
My battery box is between the frame behind the cab. I bought a pair of 4 gauge jumper cables and ran 3 cables for each positive and negative terminal (6 cables total). I was able to keep the cable only 4 feet long and still leave slack for cab movement and vibration.
USE A FUSE! As someone stated above, an inline fuse could save your truck from burning to the ground.
As a rule of thumb, each 100 watts of an inverter requires 10 amps DC. So for your 2500 watt inverter you need a 250 amp fuse. I use a 200 amp because that was all I could find when I installed it.