The first question you need to consider is if you want a coulomb counter that directly measures amp-hours in and amp-hours out of your battery to calculate state of charge, or you want a volt meter to infer state of charge from voltage and a chart with a bunch of caveats. Many of the suggestions above are volt meters.
It sounds like the OP is interested in the coulomb counter (ie with a shunt), which is a good idea. Particularly if you have solar, it is hard to interpret what the voltage means as the battery is always being charged at a variable rate during the day, so the voltage can vary wildly as a cloud moves in front of the sun. You can really only get a meaningful reading first thing in the morning.
For coulomb counters there are three basic options:
1. The no-name ebay/amazon versions (AiLi, Renogy and the like). These are the least expensive option, but have some limitations, they only provide an instantaneous value without any history or configuration, and no bluetooth connection.
2. The Victron smart shunt - which is bluetooth only, is configurable and provides history and performance metrics.
3. The Victron BMV-712 which is essentially the same as the smart shunt, but includes an LCD display to mount inside the camper.
There is a fourth option - the TriMetric, which was the bees knees 20 years ago but is now completely overpriced and out of date and not worth considering.
My suggestion (as stated before) unless you are funds limited, go with option 2 or 3. They are a bit more expensive than option 1, but provide more useful information and are the 'standard' in battery monitors. Bit if your budget is tight, option 1 will work well enough.
There is one other option, the
Balmar SmartGauge, which hooks up like a voltmeter but actually measures battery capacity. It is easy to hook up and quite accurate, but is expensive and will only work with lead acid batteries.
