What is best? That is somewhat loaded, and is almost like asking which is better, dodge, chevy, or ford...
For my two cents, two 6 volt deep cell batteries wired in series most times tend to have better amp hours and charge, discharge, recharge, low power run rates. Yet, a high quality (Gel pack or AGM) 12 volt deep cycle will work great. It all depends on what you do with your rig. Do you boondock or camp at no hook up RV parks a lot? How long is your average stay?
I boondock a lot (if not most of the time). I rarely camp anywhere with 'hook-ups'. I have run with 6 volts in a couple of my previous rigs. In my current rig I have two 12 volts, then again, I also have solar, and a generator.
As for your second question, if the temperature is not going below freezing during the night, there is no need to put RV anti freeze in the P traps. Also, there is no need to run the heater at a low setting at night. However, regardless of the temperature, it is a always good to have the unit plugged in to 30 amp household when you have it at home (especially if that is also where you store it).
There are all kinds of tips and tricks to cold weather camping. I have been known to run out on Caribou hunts in December in areas where the daily high temperature is 2 below zero and the nights will be in the teens below zero.
We are just a humble drinking couple with a hunting and camping problem.
2015 White Hawk 25BHS Glacier Package
2004 Dodge Ram 2500, 5.7 Hemi, 5Spd Manual
2008 Arctic Cat M1000 SnoPro
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