Yes, a digital voltmeter with two decimal points is only about $20 these days. Just put it on DC volts and start measuring and learning by going to the many sites like "The Twelve Volt Side of Life" Part 1 and Part 2. In a week, you will be able to answer questions when others post.
You batteries are really only good for two or three days if your fridge is running. If the rig is sitting without the fridge on they will last a week or two. If you disconnect the battery terminal on one side a fully charged battery should be good for several months if water level is good. I store my rig in outside storage with battery disconnected and have no problems when I reconnect.
