You might want to consider a DC ammeter as well. By knowing the charging current you can get a good idea of the state of charge so you know when to turn the genny off. Depending on your converter, it may take quite a while to charge the battery, longer than making a pot of coffee.
An ammeter is more difficult to install since it involves putting a shunt in the negative battery lead, and of course, 2 more wires.
Here is a link to a combined volt ammeter. You would also have to buy a shunt to use with this meter.
There are lots of other suitable meters. You might prefer a 4 digit meter. Search for "digital dc meter" on eBay.