Is that okay even if the nights are still low 40s?
The milk jugs will help to protect the tomatoes. Once the temps get below 50 degrees, they basically kinda stay in neutral, and make little to no progress toward producing.
At 40 something degrees the tomatoes won’t freeze or die or anything, but if you are at 48 degrees for example that helps to raise the temp a bit even at night to improve growth rate.
You have to leave the lid off when the sun comes out or take the jug off to keep things from becoming too hot, which could also interfer with growth/production.
Yes, they will survive even a hard frost or two. I have been doing this for many years. I don’t claim to be the best gardener, but I do raise and preserve a lot of vegetables and fruits.