We fill them water in them and let them simmer while we eat, it allowed for easier clean up. We always burned any leftover food particles in our firepit except on the day we go home. Then everything is loaded in either a plastic tote or the bathtub and taken home to wash
If you are cooking over an open fire, smear a little dish soap on the outside of the pot. The black from the fire will just wash right off.
I would get an environmentally safe soap and let it simmer with some water in the pots and make sure to dispose of the food leftovers in the fire afterwards! Baking soda and vinegar for the tough messes could help too! 🙂