Just to add to some of the other posts in this thread. Using the chart that Roy posted works well, however you must do the check when the system in not under load and has not been charged recently. Ideally, several hours of idle state. You can get one of the devices that plugs into the cigerette light plug that will display the voltage. Same thing applies, system must have been idle for a few hours to get a good reading on state of charge.
If I were you I would buy a clamp-on DC amp meter and learn how to use it. I will be a tool that you will find many uses for around the house and the RV.
Here is an example of one that is on amazon. This is cheap one, you can buy other ones that much better. But for a casual user this is probably good enough.
UT210E