The only thing I can think of that you can do is to find gas stations that have the old fashioned bathrooms. You know, where the doors are around the side of the building? Take them in their puppy purse in there. Those bathrooms aren't usually the best, but you can always bring a small pack of disinfectant wipes with you and clean up a bit before you use it. It's hard to travel alone, with doggies or children. Been there, done that! I used to carry a really small can of Lysol when my daughter was little, spray the seat, wait a minute, then wipe with paper towels, before I"d let her sit on it. She'd be doing the pee pee dance the whole time, poor thing. But better than getting some bad germies.
Good luck on your next trip
Your dogs are gorgeous!