OK so the GFCI trips. Next is to divide and conquer. As said above all six outlets need to be verified as connected in the string. Tone generator or switching off breakers should do that. Next determine the exact order in the string. GFCI should be first so test it alone. Then connect one outlet at a time in sequence, testing again at each step.
While six outlets seems like a lot... ALL other outlets need to be verified that they are NOT on this branch. Especially Fridge outlet, outdoor connector and any tucked away in a compartment. Also verify no other component such as an inverter or converter is hardwired to the circuit. If any outlets are in a slide you probably have at least one more J-box to open where the flexible cord is connected.
Always assume you have missed something and devise a plan to verify one component at a time.
Also keep in mind any recent work that has been done where a screw etc was put in a blind area.