I went tent-camping with a 5 month old, and it was very, very cold at night. Some nights it was down in the 30s. We layered, layered, layered, especially since it was never that cold when it was bedtime but got progressively colder as the night went on.
You can use a blanket as a sheet for the pack-n-play for some insulation. Works better than just a regular sheet.
For clothes, 1st she had on onesies, socks, of course.
Then, we added on long sleeve footsie.
Then we added long-sleeve sleep blanket.
Then added sleeveless sleep blanket.
Several socks on hands to act as gloves
Definitely need a cap or two to keep the head warm.
And if that was not enough, we swaddled her with a blanket.
The trick was to add layers when you needed to feed. I always added several layers at df.
Then added another layer around 2 AM or so. She didn't need to feed, but since we were camping and had to keep her quiet, I fed her (definitely was AP) but, it easy to break her of her habit once the trip was over.
Some nights if it was REALLY cold, I did take her layers off and I let her sleep in my sleeping bag. She slept, even if I didn't. So this is not recommended, but was good for last resort for those super exceptional freezing nights.
Have a GREAT time.
Mika