You must keep track of your battery voltage. This is the most basic indication of how much power, if any, is available. And it will help you keep your battery from failing very early in its life.
You don't mention if you have a battery charger, besides the solar
. If not, go get a cheap automotive one at Walmart of about 6-15 amps. If one is already built into the trailer, find out how to turn it on.
Go get a simple and cheap volt ohm meter at Home Depot and learn how to check battery voltage, or get a cigarette lighter volt meter from Amazon and plug it in. If you have no battery charger, get one and get it hooked up before your battery is ruined. You can also connect a set of jumper cables from the car battery to the trailer battery, as a test. If things start working, your batter was/is dead. If not, something else is the problem and you need help.
These things are the most basic starting points, and are critical to trailer ownership where any 12 volt power is being used.
You have to budget your power use to not exceed the available power, just as you cannot just drive your car without re-fueling it.
If you don't put fuel in the car, it will stop running. The gas gauge indicates how much you have. Same thing with the battery. The battery must be "re-fueled" to keep working. The volt meter is your fuel gauge.