Girl. Get a concentrator, holy cow. Michael has always used one and it's never been a problem (unless the power goes out, but that's what the tanks are for!)

Walgreens can provide you with a portable concentrator too, for the road, you just have to push them. Hopefully your car is tougher than my Civic, though. I tried plugging the concentrator into the lighter outlet like you're supposed to and it not only blew the fuse but it melted the connector. Whoops. They do have batteries that are good for a few hours at a time, but that's more for walking around.

For that teeny bit of O2 that Isabel needs you'd be fine driving down with just one M60 tank (one lasts for over 7 hours for me at 3 LPM, and you're using a tiny fraction of that), but I definitely wouldn't travel for more than a day trip without a concentrator. You could also talk to Walgreens about getting supplies while you're in San Antonio, they must have offices there.