There have been a few family gatherings we "got out of" because one of ours wasn't feeling too well.
I know it's a lot to consider, but maybe have it at your place, and everybody brings a dish and don't be afraid to "assign" dishes.
Otherwise, stick to your guns, and just let everyone know, "It isn't that we don't love you, but we are POOPED! We hope you understand that having a newborn is a lot of work. We would rather spend a lot of quality time with everyone and feel the best way to do that is to split Thanksgiving and Christmas this way."
Now that being said, there are things you can do to make traveling a little easier. (We live 2 hours from my family.) Let the people who's places you're visiting know that arrival time might be a little tricky because you'll need to time it around baby's feeding's. Feed her, then once she dozes off - put her in the car seat and go. Be willing to sit in the back seat during the trip in case she wakes up and needs a pacifier or something.
Take her normal diaper bag with you, but make another one that will actually be the one you take in with you.
We have what we call a "booty bag" that we keep in the car to take into places with us. When our girls were younger, we had it stocked with: a package of wipes, a few diapers, a couple of empty plastic grocery bags for diaper changes (to put the dirty diaper in), Ready to feed formula (they have 8 oz. bottles, and 2 oz. bottles at the stores), a container for some dry baby cereal (that could be put into the formula if we needed it), gas drops, tylenol, booty balm (bag balm, desitin, aquafor, whatever), bottles, bibs, pacifiers, 1 or two extra change of clothes. It sounds like a lot, but it worked out very well for us. If you don't have an extra diaper bag, a backpack will work, or a small carry-on size travel bag will work too.
ALSO, bring a cooler and any snacks & drinks you want for the trip.
It really doesn't take that long to put those things together, and it's amazing at how much it makes the trip more tolerable.
ANyway - just a few thoughts....good luck!