Very thought-provoking discussion! Yes, to be clear, I am hoping to get more than a real-time voltage reading and then just infer state of charge from the voltage. I have often used my multi-meter to read voltage, and the readings vary a lot depending on whether I have let the battery "rest" sufficiently before taking a reading.
In order to get a proper resting reading (with no distortions due to the various items that draw juice), I have to disconnect the battery and wait at least 15 minutes before taking the reading. (Some folks say you have to wait a half hour, but I have done experiments, and the readings level off after about 15 minutes).
And it is a hassle to sit there and wait 15 minutes on a daily basis -- we've got other things to do!
So that is why I am leaning toward a device that will give me a reasonably accurate measurement of how much juice is really in the battery at any given time. That's especially true since we use a portable solar panel. Since we are away from the trailer almost all day, every day, there is no way to know how much juice the panel has jammed into the battery during the day -- were there clouds during the day? Did I misalign the panel?
(Pro tip -- use the compass on your phone to find South so that the panel gets the best possible solar exposure. Don't just guess where South is. Please don't ask how I learned this.)
The solar panel, by the way, adds to the uncertainty of a simple multi-meter or volt-meter. Because the solar panel charges my battery at around 14 volts or so, the battery is often "hyped up" by the juice flowing from the panel. So allowing a sufficient rest period is doubly important, because the high inflow from the panel distorts the true baseline voltage.
(I'm sure I am mangling all of the technical lingo, but you get my point -- looking at voltage may not really tell the whole story.)
I'm going to have to review the various suggestions made above, to see which monitoring unit will do an adequate job for my limited needs.
