I like the renting an RV idea, is it possible? you bring your bathroom with you and stop all you need to.
Unfortunately the space in most RV bathrooms I've seen (in person and via Google Images) is not anything close to being enough for me to be able to clean myself after using the bathroom, for those where my butt would even fit in there in the first place (heh). There's just no way. Unfortunately, though I'd actually prefer to take the car (I enjoy long road trips usually, and it'd be cheaper) I think the best bet is going to be the plane. It's only one ninety-minute stint sans bathroom, and it'll be a large handicapped bathroom on both ends like I need.
In fact, as Rob noted, since you would be using your own portapotty, you could just rent a large cargo van (MUCH cheaper than renting a camper) with no windows and an open cargo area for room and privacy. Just an idea ... but I'd still rather dehydrate myself for the morning of a 90 min mid-day flight.
Rob - the bathroom on the plane is not an option for her - same issues of fitting in it.
I love those "life hacks" where there is something I would NEVER think of, but which, once you are told the idea, is ridiculously elegantly simple. Most of the time, I am NOT the one to see the easy fix in front of my eyes.OMG...that's a great idea, that's one of those ideas you hear and say...damn, why didn't I think of that