On my 5er, the space for the heavy 30-amp cord to curl up when on the road was covered with a panel that was attached with 4 screws. Remove those 4 screws and the panel and there was the cavity for the cord. The cord goes to the inverter, so the cavity is probably right beside your fuse panel.
To change cords, or to replace a cord that has been cut off, you'll probably need to remove the mounting screws from the inverter/fuse panel and pull it into the trailer, the same routine you'd go through to replace the inverter. Then you can access the screws that attach the incoming cord to the back/top of the inverter. So you shouldn't really need to get into the cord storage cavity if you can push the new cord up to the inverter and then fish it out of the cavity so you can attach it to the inverter.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.