An ammeter won't show how much of your batteries power is left, only how much is being drawn or charged at the moment.
A high quality battery monitor might be what you want, but they require a shunt be installed as well as a monitor. A less expensive and more practicle idea is to monitor the battery voltage. The following chart shows approximate % charge of 12v batteries. You NEVER want to go below 50% unless you enjoy replacing batteries.
I believe this thing would do the job...
12 Volt Battery Voltage Monitor