Plug it back in for 24 hours to fully charge your battery.
If this happens again you will need to figure out what is draining your
battery so fast!
It's also possible the battery was mostly dead at the dealer.
He plugged you in to do the walk thru and it gave your battery a little
charge. Now it's dead again.... This is VERY hard on your battery.
First you need to plug in and let it fully charge.
Any trailer will drain the battery in week or two if the battery is left
on. Many do not have a battery shut off
Most of us here recommend adding a battery shut off switch if you
don't already have one.
I've had to add them to every one of my 4 trailers over the years.
They guy on ebay where I buy the switches knows my ebay id by heart!