Disconnect one of the battery cable clamps from the battery. Connect an ammeter in series between the battery and the cable clamp. A reading of zero or a very small number is normal. A high number means something is drawing current that shouldn't be. If this is the case, pop/reinstall each fuse from the fuse block until you find the source.
Your battery going dead in only a few hours means which means (a) The battery is discharging internally and must be replaced. (b) There is a high current draw somewhere, or (c)The battery is very cold and you don't have enough cold cranking capacity. If you can rule any of these out, then you may have found the problem.
Be careful applying current to the small portable voltmeters... they are fused with a 10 amp fuse. You might try a test lamp first in case there is a current draw higher than 10 amps. The test lamp should not light at all.