We used to have wolfhounds and giant crosses but found their poop almost impossible to pick up with just a bag - way too soft, only a shovel, rake and supermarket carrier bag (carefully checked for holes

) would do. Now we have tiny dogs with settled insides it's no problem. Local councils have free bags available at their offices, and green spaces in towns usually have a red bin for doggie bag disposal. If you keep a lidded bucket somewhere about your bus, you can dispose of the bags when you find a bin. Maybe camp sites have them too.