batteries won't cooperate with you on most of them very much, though.
The issue is plagued by perception problems and a poor understanding of the variables involved. That means a lot of confidence in results that isn't warranted and may lead to disappointment. (it also leads to a lot of contention in these discussions as variables and context can lead to divergent experiences and mental models)
All measures of state of charge require a 'quiet' battery and proper interpretation in regards to context and equipment. There are only two direct measures of state of charge: voltage and specific gravity. For most RV purposes, a voltage measure after a half hour or so of no significant charging or discharging will provide close enough measure of state of charge.
The available energy capacity of a battery can vary by more than 10% due to factors such as temperature, cycle to cycle variations, age, and use profile. The problem with most integrating ammeters is that you have to program them with the bank capacity, which is not known very well, and then their integration of power doesn't consider all of the important factors. They provide a lot of reassuring precision but their accuracy is another thing.
For charging systems, the ammeter is nice but, again, all you have to do is to watch how the battery voltage responds to its inputs. One factor many seem to miss is that a lead acid battery needs 8 to 12 hours or more for a complete and full charge even with the best equipment.
Getting a window on actual energy use is plagued by both the inherent variabilities in lifestyle and the constraints of a battery system. Figure a typical household uses 30 kWh/day (government numbers) while the typical Airstream battery bank has a bit over 1 kWh available energy which should last a weekend. Yeah, an RV is not a house, but, still, a 30:1 ratio to start is a big jump and it means that just a few degrees on your thermostat on a cold night can make the difference between whether or not you have any battery left in the morning.
You just don't have the room or carrying capacity in your Airstream for more than a few hundred pounds of battery or more than a few hundred watts of solar so something's got to give. There's also the cycle depth issue as a reserve to consider.
Some rules of thumb to think about
: - Lead acid batteries have about 12 to 15 watt hours per pound of usable energy. - A solar system should be sized for at least a watt for each pound of battery. - Batteries need 4 watts per pound maximum charging capabilities for best health.
it doesn't seem like they're much more trouble or expense than an ammeter and shunt and they provide considerably more information.
hey, they are fun! If they don't cost much and aren't much trouble to install and you like numbers, then go for it! You do get a decent DVM and a good ammeter and those can be useful. Just be careful about the idea that having more information is going to help you achieve your goals. Sometimes it can mislead or distract if you aren't careful.