It seems to be decided randomly en route, when you hover over the caravan it says where it is headed for but seems to deviate at random. Basically if you take the quest you’re taking it as a “as long as it takes” job
If you hover over the caravan it says where they're going. The more trips the caravan makes the more money you'll get. I escorted a caravan to 4 cities and defended them once and received 6k denars.