Your truck already has it. It is built into the computer on 05 and up...possible even older ones.
Ford calls it Battery Charge Protection..it is a purpl with a light green tracer that should be just below your fuse box below your steering column.
All you need to do is send 12 volts to that wire through a switch to this wire, enage the Park Brake, have the truck in nuetral or Park if auto and it will bump to 1200 RPMs, and up the RPMs if the voltage drops.
This is the link for the PTO option, but if you read through it explains the Battery Charge Protection/High Idle.
EDIT: Sorry, had to change link, on the chart a little over half way down shows the info.
Oh, I used my Upfitter Switch #1 for mine, if you have the Upfitters.