X2 on the meter! The engine alternator provides about 14 volts. Put your meter on a DC 20 volt range, then leads on the engine battery posts when the engine is running and you should see about 14 volts. Then do the same for the auxiliary house battery and you should see the same 14 volts. If not, it isn't connected to the engine.
You can use the meter to tell how well the batteries are charged, too.
We find that engine charging of the house battery works very well if we just take a little drive every day when camping. We have no generator and we never stay at campsites with electric plugins.