In regards to burning a body to ashes, I do not think it is possible to do with a regular type of campfire. This article about cremation
http://science.howstuffworks.com/cremation.htm says that temperatures during cremation reach 2000° Fahrenheit. That reduces the tissue to an ash like consistency, but the bones need to be ground up, which results in something akin to gravel. It also takes 2-3 hours at this temperature.
According to these campfire guidelines
http://www.windsorfire.com/divisions-prevention-campfire-safety-guidelines "The average campfire can get as hot as 500°C (932°F) in as little as 3 hrs."
Based on that info, I would conclude that a campfire would not be hot enough to burn a body until it was ashes. There would be identifiable bones, which would be charred but still look like human bones.
Warning: graphic description.
In an open wood or charcoal fire, it is unlikely that all the flesh would be burned. Whatever flesh was left would act to protect the bones. The surface of the body would be charred but would still be recognisable as a human or large mammal body.
It would also take an incredible amount of wood or charcoal (way more than one 40 pound bag) to reduce the body to bones. It is possible he used an accelerant but those tend to burn very fast, releasing most of the heat into the atmosphere. They increase surface charring of the corpse but don't significantly damage the bones, unless the perpetrator has a large supply and keeps adding more.
Adding more accelerant to an open fire is tricky. Just pouring it on leads to a high risk of explosion or of the flame igniting the stream of accelerant from where it contacts the fire up to the container. Way safer is to soak something like a burlap bag or cloth tarp and then throw that on the fire but that increases again the amount of equipment that has to be hauled to the fire site.
It makes for a huge fire site, much larger than the size of a human body; say about the size of a human body with a two to three foot margin all the way around it. Such a fire site would be easily visible from the air and would look very different from the usual sort of campfire site.
The burn site would have had to have been located out of easy view of whatever road he used to get there or it would have been found by now.
While burning her body and then transporting the charred remains would help conceal what had happened to her, keep in mind that he would have to wait for the body to cool, which would take several hours even in the winter.
I could be wrong but I'm thinking Josh Powell did not have enough time or equipment to effectively burn her body. He may have made some sort of attempt but I don't think he had long enough to get anywhere close to reducing an adult human body to bones. It would have required hauling hundreds of pounds of firewood or charcoal (not exaggerating) in addition to her body.
Even if he used an accelerant, keep in mind that gasoline weighs around 6 pounds per gallon and it would take many gallons to reduce a human body to bones.
I just don't see him as having the strength or stamina required to haul so much stuff to a burn site. I could be wrong... but really, he looked like a typical desk jockey to me. Relatively young, relatively fit but not having the sheer strength or stamina needed to carry out such a task.
