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Not sure what you are trying to find here...........the brakes on your camper aren't working when you hook it up to the tow vehicle? That would not be affected by anything you can do by hooking it up to 120 VAC here. Or don't I have the correct picture of the problem? Please explain in more detail. All follow ups are included in our fee. I am making the phone chat option available for you as well.
Someone has wired this wrong............the brakes work (or are supposed to) ONLY off the power they receive from the brake controller in your truck. The only time the battery in the trailer comes into play is when the break away switch is activated and then they come on. This is for a case when the trailer comes loose from the tow vehicle. The break controller in your truck is activated when there is power to it from the truck battery. Some only work with the keyswitch on. IF it isn't that way, then someone needs to check the wiring and correct this.
And you have to apply the brakes OR do it manually inside the truck..........that is the only way the brakes are activated.
Start with finding out if the brake controller in the truck is putting out the proper voltage to the tow cord plug. IF that checks out then I would ohm out the tow cord on the brakes and see what that shows. That would take a small digital multimeter like you can find at most hardware stores for about $20 or so. Easy to use. The bottom two lugs on the cord are the brakes and ground lead. You want to see about 2.2 ohms.
You have to check the output of the break controller at the plug in the rear where the cord plugs in. Bottom two lugs. With th brakes on, it should read at least 5-6 volts or more depending on what the setting is. What is the make and model of the controller?
Is the pin pulled out on the breakaway switch? That will energize the brakes with the trailer battery and perhaps the battery is dead until you plug in the generator and supply power to the inverter.
No name on it? Most have the brand name someplace.......but, the real test is if there is good voltage at the plug with it energized.
On the back of the truck where the tow cord of the trailer plugs in............and the battery in the camper may be bad or the converter may be bad or fuses blown.........but, none of those things affect the brakes.
I have to leave for a few hours, so I will opt out of this so another tech can offer something. You may have to have a mobile tech come over and give you some help with this.
As I said in the beginning..............something is wired wrong here. Don't respond or it locks me back into the question and nobody else can resond.
I am curious as the solution on this............did you find out what happened here?