solar charge through a fabric cover

A solar cel/module will produce current in direct proportion to the amount of light it is getting. So of course it will not perform as well when partially obscured as it would in direct sunlight but it won't be zero either. Even on quite cloudy days a rooftop array will get 10% or more of sunny day output.

So pretty clearly you're getting enough charging current from the covered module to keep your battery charged with no load on it. That doesn't take much. Likely you aren't getting enough current to equalize the cells though, which lead acid battery need occasionally, so I would still give it a more agressive charge once in awhile.

One thing you might want to do is get a multimeter with clamp on DC current measurement capability. That is very handy to have in your travel toolbox so you can see what your loads are taking and what charging you're actually getting. Be sure to get one that does DC, not just AC current measurements. They are quite inexpensive now on Amazon and other online retail sites.

Flooded lead acid batteries use a sulfuric acid and water mixture as the electrolyte. Eventually with inactivity the electrolyte will stratify, meaning that the sulfuric acid, which is denser than the water tends to settle to the bottom of the cells, leaving water at the top. That is bad as it can eventually lead to irreversible battery plate damage.

Equalization in flooded lead acid batts involves agessively charging them at a higher than normal voltage for a few hours (around 15V or so). That creates bubbles of hydrogen and oxygen gasses by electrolysis of the water in the batteries. The bubbling mixes the sulfuric acid and water in the cells, reversing the stratification.

You can just equalize every month or so and as long as you replace any water that you loose from the charging you won't have any problems from stratification. If that is difficult for you to do and what you're looking for is a way to know when you need to equalize, the best way is to directly test the specific gravity of your battery cells. So rather thann to use a voltmeter which wont tell you anything about the individual battery cells you can buy a battery hydrometer for about $10 which you can use to check the specific gravity at the top of each cell. Higher means more sulfuric acid which means higher state of charge and better mixing. So what you are looking for is a consistent high sg reading.
if thats not happening then you need to aggressively charge for a few hours,then recheck. ​Dont check right after adding water as that will give you a false low reading. In your case if you need to equalize it might be enough just to expose the solar module to full sun for a couple of days by temporarily remoing the cover, you might not need to pull the batteries out or plug in the charger. The hydrometer will tell you. For most golf cart type deep cycle batteries an sg of around 1.265 indicates full charge.   

Good question on the level to take the electrolyte from for testing. If you have golf cart batteries I doubt you have enough fluid above the plates to be able to adjust the height much. As long as you are consistent in what you're doing on all the cells I think you should be fine, you're looking for variation cell to cell anyway, not really absolute sg numbers. So it's a matter of getting your technique down and consistent.

Yes battery safety is very important. It's the H2 getting produced that is dangerous, combined with an ignition source which in battery explosions is typically an internal hot spot formed when the battery has current running through it. So shut off all charge sources and loads before opening the batt caps, and give the battery a few minutes for any hydrogen to dissipate, it's much lighter than air so will vent away quickly, a few minutes at most. And by all means wear safety glasses.