voltage meter for inside camper

Voltage by itself isn't a very good way to measure SOC of the battery unless you can isolate all sources of charging and discharging for a while. But if all you want is current momentary voltage almost any cheap meter will give you a decent measurement including those you just plug into the cigarette lighter.

If you want a better meter to measure SOC consider a shunt based meter. More work and money to install but better info..