If you want the camper to be flush with the tailgate, a spacer of some sort (2x4's as you mentioned) is a viable option. My uncle did that when he traded trucks.
You will just need to be sure if the camper has extensions/compartments on the sides that fit over the wheel wells when you set the camper in, that you get the camper centered in the right spot when building the spacer for the front.
3" really isn't much to move it back.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel-Goodyear Endurance Tires
2017 Ford F-Series SCREW 4x4 - Factory Puck B&W Companion
TST Tire Monitor w/Repeater - Sinemate 3500w Gen.
Days Camped: '18=51 ☼ '19=58 ☼ '20=110 ☼ '21=60