You can only do this after "The hundred hands of Chaos" mission, as the homeless camp is unavailable until you can pass through Kenny's house.
Once you've completed the mission go back to Kenny's house and you can enter the camp via two options.
Enter Kenny's house through the door and ise Mysterion's secret entrance or use Toolshed's buddy power to clear the lava infront of the gate and proceed to the camp.
Before going for the trophy it's advisable to set up Fastpass' travel point and then proceed to the pinwheel in the background. Collect everything on the way up (One of Big Gay Al's cats is also up here) and then once finished go back down to get the trophy.
In this area you can also earn: