Freezing temps, leaving truck for 15 days, will batteries die?

I would be more concerned with your fuel gelling than the batteries freezing. If it gets below 20, you could come back to your fuel tanks full of candle wax.

 

I would imagine one or more...will be "toast". With computer drain and cold winds and all.Can you plug it in with extension cord?

 

This is a serious concern unless OP fuels up near the terminal with local, winterized, fuel. The solar trickle charger should keep the batteries up

Had a guy from the Chicago area load in Jacksonville for Thunder Bay who learned the hard way that FL fuel just won't take the extreme cold. Took two expensive days sitting in a heated shop before the fuel liquefied enough to be treated and get the truck re-started...

 

Safest bet is probably to disconnect them for that long

 

With all electrical loads removed, either disconnect the batteries or use the shut off switch if you have one. Sitting in the cold doesn’t really bother fully charged batteries.
I’ve successfully started a truck (2006 Century with a MBE4000 Mercedes) that sat for 3 weeks like that. Then when it was -22C (-07F) I had to wind it over a good solid minute before it fired up without any additional aids.
So if it’s any warmer than that you should be fine.
If it’s colder you’ll have to plug in the block heater if it has one.
If not you’ll need a propane tiger torch, fire it up, stick it inside a two or three foot piece of stove pipe under the oil pan. You don’t want the flame to actually touch the oil pan. Throw a tarp over the engine and close the hood. In about an hour it’ll be toasty warm and will start.