This is on a single splunk instance and I'm not actively using the kvstore for any of my apps so completely starting fresh with it seemed like the best choice, but even after running clean I'm still getting the same errors.
This indicates a possible problem with the underlying file system or disk, which prevented Mongo from committing a write. Errno 5 is EIO, which normally indicates bad blocks, transient or permanent file system errors. It is not typically used to indicate lack of free space, but I wouldn't rule it out, either. If this is an EBS or other network backed instance, it could also indicate network problems.