Using the volt meter, set the switch to amps. Disconnect the positive terminal of the battery, and connect the voltmeter between the battery cable and the battery terminal. If you have any reading at all, you have something draining your batteries.
If you do get some sort of reading, the next step is isolate the problem. This can be as easy as turning off all the panel switches until the reading goes to zero amps, or as complex as disconnecting wires one at a time until there is no reading. Most of the time it's an easy fix.
Be sure to check the batteries with a hydrometer. Each cell should read 1.275 and all cells should read equally.If one cell reads a bit lower than all the others, this is a good indication that the better is headed for the scrap heap. And if your battery switch in on 2 when they discharge, this indicates that one is dragging the other down and is in the process of going south.
I found that my explosive gas sniffer was dragging down the batteries of my son's boat. There was no on/off switch on the sniffer, and he didn't have a solar panel. Didn't take more than a few days to really drag down the batteries.
I've never seen a solar panel that did NOT have a blocking diode - not even the cheap ones they sell for charging through a cigarette lighter connection. I don't believe the solar panel is part of the problem.
Hope this helps,