I had the same problem with the skins burning on the inside, but lowering the temperature solved it!
They came out perfect this time, at 350° (the burned-skin ones were 400° or maybe even 450°, I can't remember). Thanks Kelli2006 for the 350° recommendation.
My sweet potatoes were huge, so they took at least 90 minutes or maybe 2 hours. I just checked them when they started to smell done and took them out when they felt tender / soft when pinched with tongs.
To prep, I just scrubbed them, poked them with the tip of a knife (about 6 times per potato) and set them right on the middle oven rack. I had a cookie sheet on the rack below to catch the drips.
The insides were super sweet and moist. The plain skins were not particularly flavorful or crispy, but they were still fine along with the rest of the potato. If I wasn't trying to not use oil, I might oil the skins next time (like Kelli2006 said), because a crispy skin would probably be better.