Looking at a picture of an 07 grand Junction 35TMS, I would guess it is either in the generator compartment mounted on the roof or a wall, or behind a wall in the front storage compartment. This is just a guess though, but a place to start. Mine is behind the rear wall of the pass through storage compartment. It was right next to the water pump, of all the great places to put the main component of the electrical system.
In case you don't know what to look for, mine is a square box with 3 big cables coming out of it. To help you find it, you can trace the shore power cable from the plug and it should connect to the transfer switch.
2013 Voltage 3905