You've had a lot of suggestions for a 1 ton van, I would encourage that as well. 6 people in an Expedition will be tight. 3rd row seating is VERY tight. Your older kids will be crammed in the middle row with minimal leg room. You will be in that vehicle for a long time over 2 years, it will get old quickly.
A one ton van, even a couple years old, will provide WAY more room for kids to stretch out. The kids in the backseat can get in and out easier. The kids in the middle seat can stretch their legs. 3 kids can sit in the back row, one in the front passenger seat and one of the parents can stretch out in the middle row. On top of the extra space you'll have, you will have plenty of truck for hauling. That leads me to my second point...
A light weight trailer is meant for occasional and light use. A year with 4 kids is neither occasional nor light use. You will NOT be happy or comfortable in a light weight trailer for that long of a time. I would encourage you to buy a "regular" trailer with a traditional bunk house with opposing slides. That regular bunk room will get tight, especially with 4 kids in it. A bunk house with opposing slides will allow more floor space for them to sit and read/play/watch tv and get ready, without being all over each other. Again... this isn't 3 or 4 days at a time... this is 365 days of use.
A 1 ton van gives you room for the kids to have their own space while traveling and gives you a more stable truck for towing. A "regular" trailer will last longer with the wear and tear that full time living with a family of 6 will inflict on a trailer. A bunk house camper with opposing slides will give significantly more room for the kids.
This can either be the experience of a lifetime, or a disaster that the family will hate. You'll be spending decent money to do this trip... do it right. Our family took a 6 week trip cross country when I was 13. It was in a half ton van and 26' bunk trailer with no slides. The majority of the trip was annoying, but getting to experience New Orleans, Vegas, San Diego, San Francisco, Big Sur, Pacific Coast Hwy, Yellowstone, Montana, Tahoe, etc was incredible. I plan to take my family on a cross country trip when I retire in 7-8 years, but probably only 2-3 months long. If its done right, it will be a great experience. Do it right.
