Here's a link
to the diagram for a 2008 chassis, taken from the Ford Body Builder Layout Book (PPLB) site
. The trailer turn/stop lamps are fed from Fuse 26 in the Power Distribution Box, which is under the hood. It's a 20A fuse.
Sometimes the BBLB pages include a drawing of the PDB and the fuse box inside the coach. The pages for 2008 don't. So, you can download the owner's manual from this Ford site
. The fuse box info is on pages 194-200 of the 2008 E-450 manual.