The isolated batteries/inverter idea is a good one; you can also put a timer in the cord that will turn things on, just like it was plugged in at home.
I only buy NEW factory appearance blemished batteries (think "scratch and dent"); I get Group 30/31 for $38, all being big-name brands; I have not gotten a bad one yet; they last as long as the $98(or more) batteries I used to use; you could go this route and not spend a bundle.
The thing to do is PERMANENTLY mount these extra batteries, wired through a good isolater, or switch, and you can use them year round for all kinds of 12-volt accessories, auxilliary lights, and such.
Mounted permanently, no one is as likely to help themselves to some free batteries.
There are automatic starters that will fire the engine, idle it for fifteen minutes, about every couple hours.
You can stick a cheap generator in the bed, crank it up an hour before leaving.
If it is getting 30-below, I doubt any of the un-plugged gas-burners are going to start either.