how many leaving fridge overnight?

Depends on the draw and how good your batteries are. However, I have a fridge in mine and leave it running all the time, except when I'm on hometime. Never a problem.

 

My truck would shut down power to non critical systems when the batterys got too low. I've lost some food that way but, my truck would always start. Luckly for me.

 

I leave mine on all the time. Never a problem.
If you're getting down to 12.1, you're batteries are getting weak. Need to charge up again.
Draw is the amount of wattage you are using. Just as a basic understanding, If a battery is rated for 100 amps, that will give you about 50 amps max you can use. Really 40 is closer to the limit.
40 amps X 12 volts = 480 watts available. If your fridge draws 125 watts per hour contineous, then you will have used up your 'bank' in about 4 hours.
Fridges don't run contineous, batteries have more reserve than 100 amps and you have multiple batteries, but that gives you the analogy.

 

less than a year driving experience and you bought a truck congratulations. I really hope you make it as an o/o. As far as the Fridge if your batteries are ok it should be fine for a 10 hour break but any longer i'd unplug and keep the door closed.

 

It means either your batteries aren't getting fully charged or they are getting old and won't take a full charge anymore.
Should be around 12.8 - 12.9 ten minutes after shutting off the engine.