How do I prevent burning the bottom of the bread when cooking over a campfire?
Many camping cookbooks recommend picking up some coals from the fire and putting them on the lid of the dutch oven while it is on a grate above the fire. This gives a more "all around" heat rather than just trying to cook the whole loaf of bread from the bottom. This image search gives lots of examples.

Personally, I don't try to bake a whole loaf of bread - I make English muffins. Quicker time to edible food, easier cleanup, you don't need to carry a heavy pot since the frypan is coming anyway, and if one burns the rest might still be ok, so less risk. (Important when you're feeding your children in the wilderness.) I wrote it up with pictures long ago - see if that helps you.