A battery monitor will tell you amps in and out of your batteries, state of charge in percentage, and time left at current usage. You don't want to take the batteries below 50 % and a voltmeter will not tell you state of charge in any immediate way so really can't be used for this. Here's a link to Xantrex Linklite. Xantrex Technology Inc. - Boats - LinkLITE Battery Monitor - Product Information
TRhere are others from Xantrex as well as other companies. The Victron BMV 600 is highly rated and $175 at Jamestown.
Victron BMV 600 Battery Monitor