You are right regarding getPowerlevel function. It is a mistake in the documentation, because this function is not included in current API version. It was developed for older versions. We will fix it ASAP.
If you want to know your actual power level, there is a global variable called "powerlevel" storing value.
Regarding your RSSI issue, from DM guide:
From DM guide wrote:
---It reports the Received Signal Strength of the last received RF data packet. It only indicates the signal strength of the last hop, so
it does not provide an accurate quality measurement of a multihop link.
Therefore you probably are getting RSSI levels of the closest mote.
About ranges mentioned in our documentation, they are extracted from DIGI's documentation. They provide this ranges in the best conditions, so probably you only achieve smaller ranges. No matter what channel you use, just avoid obstructions between motes when possible and place motes as close as possible.
Hope this is useful for you. Let us know your comments.