Use a digital voltmeter to test for voltage differences.
Start on th 20 volt scale, put some equipment, lights etc on to put a gool load on the
battery.
Now put the test leads on the positive on one
battery and again on the positive of the other, you shouldn't get a reading, reduce the voltmeter down to 2v scale, still shouldnt get a reading and finally to the 200mV range, if you get a significant reading doing this test you have a high resistance between the
battery terminals, (poor joint, connection or cable too small.)
Do this again on the negative terminals of the
battery.
If you do this again from the negative terminals to the vehicle chassis, any voltage here indicates a resistance in the Earth return lead, mine had 0.75 volt drop.
If this is happening, you need to increase the earth, I took a lead from
battery negative and bolted directly to the body.
You should also extend your voltmeter lead (any wire will do) to measure any voltage drop from the vehicle
battery negative terminal to the leisure
battery terminal.
The earth to the chassis is not always 100%
If you measure the voltage across the vehicle
battery with the engine running and then across the leisure
battery, they should be similar, any difference is caused by the losses of the cables and
fuse. This is what you are trying to eradicate.
Remember, if you get losses in the leisure
battery feed cable there isn't enough voltage to fully charge them. The flatter the leisure batteries, the greater the voltage loss on the cables as a result of the batteries trying to drawer more current, resulting in the leisure batteries not getting enough power to charge before the alternator has charged the vehicle start
battery and reduces output.
I found this out over a couple of weeks tour, the leisure batts (2x110AH) got more and more discharged.
Hope this helps