The cheap and easy way is to get an inexpensive digital voltmeter (roughly $20 for a decent one), then occasionally measure the voltage at the battery or at one of the 12v receptacles. You can also look for a 12v voltmeter that plugs directly into a 12v receptacle.
Fully charged, you should be at about 12.6 volts. Don't let the battery get below about 12.0 volts and you'll be OK.
If you want your batteries to last longer, paying attention to how they are charged will make a difference. Occasionally checking specific gravity and water levels is important.
If you want, you can make this really complex and expensive, with hundreds of dollars in chargers, gauges and meters - but in reality, watching the voltage and specific gravity are the two most important things to pay attention to, and $30 will get you everything you need to do both.