Thanks again everyone! I'm back now, and I'm amazed at how well the trip went overall. I had a rocky start: I had a short flight (~2 hours) and felt fine that night, and the plan was to depart by car the next morning. But the next day I was SO sick I could hardly leave the bathroom. I'm not sure whether it was perhaps food poisoning or similar, as opposed to gastro symptoms from a mast cell reaction in response to the flight/travel, but either way it was crummy.
It took most of the day to get back to normal, and so we left at 9 pm instead of 9 am, and drove only 70 miles the first night, but other than that it was a fabulous trip! Things that made it successful (in case this might help others in the future):
-traveling with good friends who you're close enough with to discuss the state of your bowels without it being weird
-travel with people who don't think it's a pain the neck to pack your own food for a long trip. My friends keep kosher and gluten free, so they are no strangers to adapting to specific dietary needs. We packed several coolers of healthy, familiar food (including 16 hard boiled eggs, which the three of us finished off just hours before reaching our destination... wow!) and kept ice packs in them during the day and put the food in the hotel 'fridge at night. No iffy foods/no rxns = priceless. I'm not sure I would have felt as comfortable being the weird one who packs all her own food and doesn't want to go to restaurants if I had been traveling with people I didn't know as well.
-extra meds. I added allegra in the morning and an extra zantac during the day. I actually had fewer minor rxns than usual, even though we were traveling in an old Toyota with 180,000 miles on it that vibrated like crazy. (Vibration is not my friend.) We drove 2,500 miles, vibrating like mad the whole way, and I really didn't feel that bad. At home, a trip across town on a bus with bad shocks is sometimes enough to ruin the rest of the day, so this was a huge success.
-rest when needed. Again, I was really lucky on this. I think I felt better on this trip than on a normal day, since anytime I felt a little peaky or POTS-y, I swapped out as driver and took over the reclining front seat for a couple hours. On a typical day I don't often have that flexibility to lie down when I need to.
-clean air? I'm from the east coast, and the air pollution and humidity are pretty rough on my asthma. I have to admit I'm sort of fantasizing about moving to Wyoming after experiencing the air there.
All in all, it was (surprisingly) successful! Thank you again for your responses!
ETA... PS: There's no way I could have done this trip a year ago, pre-diagnosis and pre-meds. And the diagnosis and meds happened because I found a good doctor through THIS forum. So THANK YOU for that as well! It's beyond amazing to sometimes be able to do "normal" things again.