-10 real temp or with wind chill? Doesn't sound that out of whack to me. I know that when I hit 10 degrees and below I plug in whenever I can. I can typically fire off up to 0 degrees sometimes less, but the starts are ugly and can take 2 times on the plugs.
If your batteries tested fine I'd check cable ends. Grounds and Positives. And as already said check the starter even though it's new. These starting systems are tough on the electrical system.