gages are very inaccurate, as waste piles up on the 'sensors' in the tank and shorts them, giving strange readings. My black tank in particular has shown half full for two years now.
Always drain the black tank first, then grey, as that flushes out the nasty material from your lines. Both tanks drain through the SAME exit point and through the same hose, which I think is a 3" hose. If I'm wrong, someone will be sure to correct me.
This site gives great info, particularly for the new person. Most of what I know, I learned right here.