Cooling System Leak

I hope this is the correct place to post.
My daughter has an 05 4.8 that has started drinking coolant.
Can't see external leaks, so I'm assuming it's a head gasket, cracked head, or block.
The fresh air garage is not kind right now, below 0F, is there a sealer someone has used, or might recommend, to at least make it to warmer weather?
I'm concerned about plugging the steam tubes.
Thanks in advance

Put a pinch of Copenhagen in there.

I'd check water pump first. Those things leak out of the weep hole and usually won't show that it's leaking.