The most comfortable car for long distance driving is the car with the best seats, and that means Volvo. However on the reliablity and cost of repairs Volvo goes to the worst of class. Toyota Camry and Honda Accord seats are pretty bad for long distances, but they are best in class for reliablility.
Something in the middle would be a used BMW. Good seats, pretty reliable, and some fun factor is nice to help make all those miles more enjoyable. If Beemer’s are out of range of your budget, take a look for some Saabs. Very comfortable seats, good fun factor, and perhaps reliable.