The big question to me is how comfortable your wife is with the idea of travelling alone, and from your post, it sounds like she has the self-confidence, adventurous spirit, and driving abilities to make the trip quite well. Why spend all of your vacation time driving if you don't have to? Your wife and son will probably have a great time, and you can meet them in the lower 48 and spend your vacation time actually vacationing instead of driving.
My husband is driving from NH to MN (in a 2001 Suburban towing our 2005 Airstream) this summer by himself with all 6 of our children, ages 11 down to 1. He is a teacher and has the time off, and I am an RN and don't have that luxury. I am saving up all the vacation time I can so that I can take a road trip to AK with the family in a few summers. My girlfriends are amazed that my husband would attempt such a trip with all 6 kids by himself, but (and this is the important part!) he has NO qualms about it and is looking forward to the trip. They will be gone at least 3 weeks, and I know they are going to have a blast visiting his family in MN.
When my husband was a child, his mother and father used to drive their 10
kids in a station wagon and towing an airstream, all the way from MN to Mexico City. My father-in-law would then fly back to MN while his wife and 10 kids stayed in Mexico for a month or more (he ran his own business and didn't have that kind of time to spare), and he'd fly back down to help drive back at the end of the summer. It worked quite well for them, and my MIL never had problems staying down in Mexico by herself with the 10 kids.
I say, what's good for the goose, is good for the gander. If your wife wants to do it and has the skills and her wits about her, why not? And like the others have said, once she has completed this trip, BOTH of you will have confidence that she can Airstream anywhere on her own.