A multimeter is usually limited to measuring 10 amps or so and requires disconnecting the positive or negative terminal from the battery and inserting the meter into the circuit. Because 10 amps is relatively low current, it is likely that you may blow the multimeter fuse. With the multimeter set to volts, you cannot measure current.
Your best choice is to install a Baterry Moniitor. These typically use a current shunt to measure current going into the battery and coming out of the battery. It adds up the current readings over time and also determine the battery's state of charge. Energy being supplied by the battery to the various devices subtract from the state of charge and energy going into the battery from solar, truck alternator, and shore power add to the state of charge. This gives you realtime knowledge of how much energy is stored in the battery at any time.
You get both the current going to and from the battery and the total energy stored in the battery. You will be able to determine how much longer you can remain camping with a cold fridge of if you need to find shore power to keep your food safe.
There are several makers of these devices. One popular among the forum members is Victron.
Paul
I thought getting old would take longer.