What generator do you have? If it's the 60 amp you'll definitely need a voltage converter. If a dual voltage unit, you can hook to the rear battery for small loads, but they won't be switched with the ignition.
The Orion 70 amp 24-12 converter has a built in relay, so you can hook it to the batteries, then wire the relay to any switched power source. With a 3 speed truck that would probably be the fan controls, in a 4 speed the transmission relay input behind the battery box.
You can also buy USB outlets that work off of 24V input, and wire them into a live circuit of the truck. if only USB they should be low enough draw as to not cause problems. One favorite place to wire in small loads like this would be the horn circuit, as it's only live when the light switch is in service or drive positions.
