To me, it sounds like you shouldn’t worry about a voltmeter right now. Usually, a problem like that is going to be one of two things: a bad battery not holding a charge, or a short drawing enough current to kill the battery.
A parts store can usually recharge your battery and load test it to see if it’s any good
If you have a short circuit, start by using an handheld multimeter to measure current, and narrow down the circuits by pulling fuses one at a time until the current drops. Current with everything off and not running should be under 0.1 amp.