We have to be very careful here . Sadly there are many seasoned caravanners who have absolutely no idea how things work. Same with cars. Today you almost need a degree in microelectronics to change a speedo bulb( not bulbs anymore).It's part of the 12v (not 230v) system, so it's a simple matter to check of there is current to the back of the socket with a multi meter. It's no different to checking if there is power to a tail light on your car, would you pay a garage to check that? .
I agree, being one of the Internal IT guys for our company I had a recent "issue" with a user complaining that the USB port was not working on his laptop as his iPhone would not charge. He had tried with 3 cables but not working so it MUST be the port!Going back to the OP question, in my experience the USB sockets rarely fail, I have found the cables used to connect to them often fail, so in the first instance I would try another cable.
You probably know this, but check the port with a magnifying glass to make sure that non of the pins are bent which can cause a short or bad connection.I agree more often than not its the cables that fail, though just last night I plugged my android pad in to charge, only to find this morning it hasn't, and it definitely is the micro USb port that's loosing contact with the PCB, I will have to try and open it to see if its a solder connection or worse.