keep the knob pulled out for normal operation , if storing for a period of time without shore power connected ,push in to disconnect the battery otherwise you will drain the battery from parasitic drain.
Keep in mind the inverters are bought bulk and cheap to the manufacturer ,the more load you put on them the harder they have to work, hence little dimming and maybe flickering.
You could all ways check the connections to make sure all is tight, CAREFULLY or with all power disconnected and tested.
If you plan on keeping the trailer you could all ways up grade to a better inverter.
You could all ways pull the panel cover off and see if there's any 12v wiring to those fuses . No wiring /no fuse ect, if there's wiring try a fuse and see what it works and label it.
2011 F-350/CC/LB/Drw/6.7L
2013 Cedar Creek 36 CKTS "Pearl Edition" Trail-Air Pin Box
Pullrite 20.5K Super 5th