Jim, monitoring voltage can be a pefectly good way to monitor batteries. They will pretty much range from 12.6 to 11.6 volts as they go from full to empty, with some load on them. When they are freshly charging they will show a false high (i.e. 13.6) but a minute or two of a load will knock that back down to reality. Likewise, engine starting will pull them down very low for a minute.
So, if you find out what your batteries are supposed to read (they may vary by .1-.2 volts depending on chemistry, etc.) and any similar variation from your meter or wiring...you can get used to seeing "this means full, this means half empty, this means I'd better start the engine while I still can".
Group 24's are best left for an overnight or starting the car. Groups 31's are almost the same price as Group 27's and still luggable by one human.
Battery chargers are not 100% efficient, they waste about 10% of the power they are trying to put into the battery. Once you have a charger than needs more than 10 hours to recharge your battery--the waste goes up a bit more, because the charger just isn't "pushing" hard enough to be efficient.
So, if you had two 100AH batteries, and decided to be religious about just drawing them down to 50% (using 100AH total)...then you probably could use a 10AH charger for 12 hours, and it would be "just enough" to recharge them. Upsizing a bit more would be a good idea, if your budget allowed it. After all, you might draw them down more than 50% some time.