I don't have a Sunseeker but my fresh tank capacity is also listed as 50.
After running down to where the pump was sucking air and the little red lights were still showing 1/3 tank I put a bucket under the fresh tank drain and
got out 9 more gallons!!
My pump suction fitting was in the lower side of the tank but the drain was in the bottom.
I removed the drain fitting and replaced it with a T and put pump on side of the T and drain valve out the bottom.
9 more gallons of USEABLE fresh water.
That's enough for another navy shower!