Awww. I'm so glad you're going to take your old girl on a road trip. It sounds like you'll both enjoy it and make lots of good memories with her.
And I agree, putting a dog in a cargo hold is stressful for both owner and dog, no matter the animals age. It can also be unsafe, depending on weather conditions.
Could you arrange for a private rental? Like, do you know anybody who owns a spare car who would let you borrow it for a month for $500?
I'm not sure where you're driving to/from and how flexible your schedule is, but since you have some time to plan ahead, you may be able to find an auto transport company who would need a vehicle moved from your area to your destination. Some companies hire non-professional drivers. I believe these services are called "Driveaway Services" and maybe you could get at least one direction covered that way?