I just change on the side of the road, using my car as a block. There's no natural cover out west (tumbleweeds just don't cut it) and rarely a rest stop or porta potty. And the cars are going by so fast... no one has time to see much anyway. I don't worry much about the top - I leave my sports bra on and put on a dry t-shirt if it's going to be a long drive home, mostly for the fabric in my car's sake.

And you could always accost others nearby. Once I asked a nice older fellow if I could slip into his motorhome really quickly to use the potty. He totally let me and was really nice about it! (Probably unsafe but... I really had to go.)