couple of suggestions based on my portable solar setup.
1) use anderson connectors at the trailer end and at both end of any extension cords. very very reliable and easy to use and plug unplg. just make a short mc4 to anderson adapter to go between the panel and cord.
2) rather than the mc4 extensions, two options. One is to go buy some 25 and or 50ft extension cord, preferably 12ga, OR even better, buy or find some 30A trailer cords with bad ends you can make into extension cords. again with anderson connectors on both ends
3) to minize resistance loss, at both ends of the cords solder the ground to the neutral leg of the cord. this will make a 12g into 10 g, or 10g into 8 ga reducing resistance loss and using the extra wire.
4) I've found 25ft to be about the minimum extension length I've used. and I've used as much as 150ft. you may be suprised how often you need 50ft.
5) consider using a MPPT controller and panels in series. mount the controller in the pass through or near the batteries. this will minimize resistance losses and maximize solar harvest. One inexspensive controller is the ecoworthy. I happen to be using two 30A midnite solar KID MPPT controllers in parallel with the controller hooked to my 480W of roof solar as master and the other controller connected to up to 3 160W portable panels.
Gives me up to 60A of charging in good sun. and either one normally can keep me well charged. If we are parked in reasonable sun, rely on the roof panels, in shade portable.
and if I want to run things like the icemaker and dehumidifier off the inverter, with both set up I can do it pretty reliably.
