Oldcamper - Hi and welcome to our group! Testing a converter is easy but you need a multimeter to see if it is charging the battery. They are available at home improvement stores if you don't have one and they don't have to be expensive.
A converter serves two purposes:
1. It passes 120Volt AC house power to the wall outlets and appliances that can use it, such as the refrigerator, hot water heater and microwave, etc.
2. It "converts" that AC power to battery charging DC power. The battery then supplies power to raise the top, light the lights, run the water pump, etc.
So, to test, simply plug your trailer into a source of house power. You will probably need a 30-20 Amp plug adapter to connect to your home's outlets. At a campground, this is known as "shore power", and they use 30 Amp outlets so no adapter is needed.
With it plugged in, all you have to do is plug some home appliance or light into one of the outlets in the trailer. If it works, you are delivering 120 Volt power to the trailer. You could also use the multimeter to see if an outlet is "live" with 120 Volt power.
Take your multimeter to the battery in your trailer and see what the voltage is at the battery terminals. If the converter is charging the battery, the multimeter should show something between 13.5-14.5 Volts across the battery terminals. If it is showing 12.7 or less, you are NOT charging the battery and you may have one or more blown fuses in the converter. I've had to replace the ones in mine because they blew.
That's really all there is to it. Nothing to turn on.
If anything I've posted is unclear, don't hesitate to ask me to clarify. I've no knowledge of your electrical expertise.
- Jack