Battery draining...HELP!

[784X4GUY]

I just charged my battery and put it back in the truck and it started up like a champ. Well the next day I went out to start it and well battery had drained down a little bit and well motor would barely spin and so it wouldn't start. I checked the voltage and it was at 11.2 volts. But the day before after I took it off the charger it was sitting at like 12.6 volts and all day it was sitting around that. So I am thinking something is draining the battery overnight. What could I do to track down what could be causing this? Also when I turn the key on to go to start it my voltage gauge will go up to about 10 volts stay there for a second and then shoot on up to 12 volts. Does everyone elses vehicle do this or is there something wrong? Thanks everyone for any help...Mike