First and foremost, you need to have a fully charged battery or batteries installed, connected and the battery disconnect disengaged (if your year has one, my 2019 didn't).
I would not try to operate any high load electrical item like slides, tongue jacks, or leveling system (if yours has one) without batteries. Putting that much load on the converter alone while plugged into shore power (the heavy electrical cable to a fixed power system at a home or campground) will lead to early failure of the converter. A converter converts 120v AC power to 12v DC power primarily to charge the batteries, but also to provide a secondary power for most of the items in the camper like slides, tongue jack, lights, etc.