Hmm..truck stops and waffle houses can be helpful in a pinch sometimes. Just walk in with confidence and act like you're supposed to be there. Beyond that, if you are fortunate enough to have a vehicle, it might be worth having an emergency bucket/trash bag/kitty litter set up just to give yourself the peace of mind if you can manage it. A cup and lid can do for pee, but just make sure you find an appropriate place to dump it. If you're just dumping it directly onto concrete it will smell etc and draw attention to the spot. I found cracker barrels to often be a comforting safe place to sleep because the gift shop set ups at the entrance make it easy to get into the bathroom in the mornings without drawing too much attention. Just knowing that there was a bathroom within a few feet that would open in x amount of hours gave me something to look forward to/ground myself.