This Web site is a great place to start:
The 12volt Side of Life (Part 1)
There are diagrams on that page showing how to hook up 12V(and 6V) batteries. Make sure the batteries are correctly cabled together.
As "StuartN" said, the 120V and 12V systems are separate, except where they come together in the converter (assuming you have one). The converter will be connected to the power center ("breaker panel"), either with its own circuit breaker or by being plugged into a 120V outlet near the converter. If you shut off the converter's breaker, or unplug it, you can test the 120V side and the 12V side separately.
Shut off all the 120V breakers, then plug the shore power cord into an outlet. Turn on the main breaker. If nothing makes noise or smokes, turn on a breaker for a 120V-powered circuit. See if that circuit works. Check each breaker in turn. If they all work, the AC is probably OK. Shut off all the 120V breakers.
12V circuits and devices should work if the cables between the batteries and the DC fuse panel (in the power center) are intact, wired attached correctly to the batteries, and the fuses aren't blown. Turn on a DC-powered device. If it doesn't work, find the fuse that is in its circuit. Is the fuse blown? Get another one, but don't replace it yet. Check to see if there are "master" fuses or circuit breakers between the batteries and the fuse panel. Are they tripped or blown? Yes... THE STOP RIGHT THERE and check the battery cabling again! That's because these "master" fuses or circuit breakers protect the battery cables from damage, and also protect the circuits and devices attached to the fuse panel. If you just replace a master fuse or reset a DC circuit breaker without fixing the problem, you could damage the 12V wiring and devices in your trailer.
Enough info for you to get started.