If you are worried about running out of fuel riding in unfamiliar areas, you can get a spun aluminum campers gas bottle for a few bucks.
It only holds 1 quart of fuel, but it also takes very little space in your bag or trunk. If you can get 40mpg riding at 55mph, it will take you 10 miles down the road. It would take you 3 hours to walk ten miles, so its better than nothing. Think of it as your secondary reserve tank.
I think I have to hold up the other side of motorcycle road trips. You can ride 700 miles in one day on a motorcycle. It will be more of an ordeal than an enjoyable cruise on your bike.
My personal experience with a windshield and lowers and a trunk that is an excellent backrest, after only an hour on the bike I need to stop and get off and walk around for a few minutes. In a car you can move around and adjust your seat and move your feet, but on a bike you are in one position all the time. If you push on and your legs get numb, you got a real problem because now you cannot stop and put your feet down. In a car I can drive 80mph for 3 hours at a time before I feel like I need to stop for a break.
Add to this the fact that you will have family expecting you and it makes your road trip more stressful. When you are touring by yourself, you can go where you want, take a break when you want, and if it takes you two days to get from A to Z, it doesnt matter. But with people waiting for you at the other end you will feel pressured to push on, even if you have pushed yourself too far.
I get the Great American Road Trip completely, and Im hesitant to type this out loud, but... sometimes its better to take a car.