And for those of you too lazy to go to post #31, here it is:
The specs state that, for a '78 Sovereign (31') center bath, capacities are 23 gallons main (Black), and 13 gallons aux (Gray).
For what it is worth, I measured (in a one gallon container and a five gallon bucket) how much fluid each tank took.
The main (black) tank took 30 gallons and the water level was lacking about 1/4" from being at the bottom of the toilet extension pipe, the shower pan quit taking fluid (gray tank) at 23 gallons.
Interesting to note that he measured slightly differently than the specs. Also to note is this is for a mid-bath model. If you have a rear bath one, it WILL be different.
I recommend his measurement method. Just 'cause.