You can certainly prove charging or not by putting on some light bulbs in the fifth wheel and having someone watch them. The other person starts the truck. The increase in brightness is very noticeable.
On the plug receptacle on the truck, you should be able to identify the charging positive and the negative terminals, and connect with probes onto your voltmeter. Again, have someone else start the truck while you hold the leads onto the correct terminals.
If you have it at the plug, move to the trailer and directly to the battery terminals and see what the voltage is there with no truck, then with the truck running.
It is a process of elimination. The next step is to identify whether you have power on the positive lead and no negative return, or no power but a good negative return. Again, it could be either the positive feed is gone, or the return is gone. Take a good negative with your jumper cables from the truck to the negative on the trailer battery. If that helped, it's the negative line and not the positive. If it didn't take a good positive from the truck battery to the trailer and make sure the voltage is now the same on both batteries. If that helps, it's the positive feed that is missing.
