Has anyone ever driven across the country with an oxygen dependent passenger? Any experiences with routes through high elevations?

No, but here are some elevations:

I-90: It's about 6,350ft just east of Butte, MT.

I-80: About 7,100ft at the Continental Divide between Rawlins and Rock Springs, WY. And then ~7,200ft at Donner Pass west of Reno, NV.

I-70: Eisenhower Tunnel at just over 11,000ft; then Vail Pass just under 11,000.

I-40: ~7,200ft at the Arizona Divide west of Flagstaff

I-10: ~4,600ft at the Continental Divide west of Deming, NM. I think that's the highest point.

You MUST confer with his/her doctor about this. A lot will depend on just how dependent the passenger is, how frail, what the reasons that oxygen is needed and what would be the difficulities should he get into trouble.
One thing that you do NOT want to do is to take the advice of someone that does not know the exact situation. What might be OK for one person, could be disasterous for another.

Oh, actually, I think I-80 gets to about 7,500ft just east of Evanston, WY.

Thanks for the replies but I have the elevations and wondered if anyone has experienced oxygen use at these various heights. Of course, a Doctor has been consulted but would like to hear if anyone has had firsthand experience.

You will also need to arrange to get additional oxygen/refills on your trip - this is sometimes more complicated than you would think, since many companies do not want to service/fill anothers equipment and most require an MD order to issue oxygen.

Also, make sure you have enough related supplies - extra tubing, nasal cannula, connectors, etc since you might have difficulty finding compatible supplies on your trip. That can raise all sorts of insurance issues, since most insurers only will pay for/approve a certain number per month. If passenger does not already have portable refillable tank, this is a good time to look into that - don't want to be wheeling a heavy green tank everywhere - go for shoulder pack. That also raises issue of refilling this, since most I know of are filled from huge cannisters people keep at home in their closets.

If by chance person uses BiPap or CPap mask, try to get extra to take, or if not an entire extra, some tubing, elastics, etc. Again, do you want to be in the middle of nowhere at a high elevation looking for supplies.

Since even humidified oxygen is really drying, make sure passenger drinks extra fluids - which may be a challenge since then they may worry about bathroom availability in remote areas. Sometimes people complaining about effects of altitude are actually suffering ill effects of dehydration.

Not trying to be negative, just giving you some fixable issues to think about before the trip.

PS - sorry - I don't have an answer to your real question!