YOu said "The main breaker at the meter" This .. is very strange,
I would like to see a photo of the breaker. but instead I will tell you what I'd be looking for... A button on the breaker marked TEST.
If there is such a button,, What you have is a groung fault. You said you ran 50 feet of 10/3 Ok I assume on the end of that you put a TT-30 outlet, which is laying on the ground with your trailer plugged into it.
That is the problem, moisture is tripping the GFCI side of the breaker
IF.. that is... It has that test button.