If you want accuracy you want , as said , to install a battery monitor not just a volt meter . Something like a Trimetric TM 2030 -RV .this has been the go to for years but there are others Volt meters are not very useful in determining battery Heath. The monitor can be programmed with total amps in the bank and then counts all amps in and out. After 24 hours it will be a accurate battery level meter.
Batteries have a wide range of voltages even fully charged with a substantial load will show volts in the mid- low 11’s the only way a volt meter is helpful in battery state is a battery that is “ off line “ resting with no load on for a couple hours.. not practical in a rv that’s in use.