Try turning the ignition on WITH the battery tender attached. Don't start the bike, just see if the voltage still drops. If it works, your battery is hosed. If it's that new, you may have a short somewhere that's grounding it out and killing it.

 

Possibly the R/R is junk, and shorted out your battery, taking it with it? I'd pull the battery and take it to a shop, have it charged and load tested, if thats an option. Its still sounding like the batt is junk.
But I'd definately do a good diagnosis on the charging system to find the culprit.