Don't call the professional yet. Since you have a meter do a few things. They're all easy and quick so what have you got to lose? :)
Disconnect shore power.
Check across your battery for battery voltage (write the voltage down). If you have voltage then continue.
Grab your negative battery cable right at the post. BTW it should be white :). Using your hand follow the wire until it goes to the frame. If the wire on the negative terminal does not go to the frame you're hooked up incorrectly. Find the wire that does go to the frame and put it on the negative terminal.
Any wire that does not go to the frame is probably positive. Follow each wire until it goes into the trailer. If there are any bumps or rectangles in the wires you have probably found a circuit breaker.
Check for battery current from each post of the circuit breaker (CB) to the negative battery post. If you don't have voltage on each side, the breaker is tripped or bad. Anything less than battery voltage on either side is bad. Check for a small reset button on the side of the breaker.
If you find a fuse careful inspection will reveal that there are two holes on the back of the fuse, one on each side of the amperage rating. (silver dots on either side of the 10 in the picture below).
These holes allow you to probe the fuse for voltage without disconnecting it from the holder. As with the CB above check for current from each hole in the fuse to the negative post of the battery. Anything less than battery voltage on either side is bad (either fuse or holder).
If everything checks out fine then temporarily plug into shore power. Now go into the trailer with your meter and find a good ground. You're looking for a place in the fuse panel where you can put your positive meter probe into the back of a working fuse and touch the black (negative) probe to ground. If you get 13+ volts you have found a good ground.
Okay now go unplug the shore power cord and go back into the trailer and check each reverse polarity fuse (green 40 amp fuses) to ground. Check both holes on the fuse to ground. You should get battery voltage on each hole on the fuse to ground. If your battery is connected you will get battery voltage on at least one of the two holes in each 40A fuse. If you get voltage on only one of the two holes then your fuse is bad.
If you get no voltage at the green 40A fuses take a digital picture of your fuse box. Look at the picture and make sure you can see where every fuse goes.
Go verify that you are not connected to shore power.
Now pull all the fuses out and remove the panel that covers the wiring behind the fuses.
Look for a thick black and white wire near the fuses and connected to the circuit board that the fuses are plugged into. You will probably see markings on the board for + or POS and - or NEG. Use your meter to verify that you have battery voltage. these are the wires coming from the battery.
If you do not get voltage there is a problem with the connection to the battery. At this point you'll probably either want to call in a professional or go check at the battery one more time.
