Experience points you gain for finishing Caravan rise with Your level.
Best Way to level up id hunting centaurs and floaters on desert for some time, until lvl 30-40, then start caravans, from about lvl 100 caravans start to be most efficient way to level up your character.
There are obviously better but very boring ways to level up faster like junk recrafting, but they are not something i would suggest to new player.