spoon059 wrote:
The more likely theory is that the OP is using an incorrect method to fill his black tank or simply is underestimating how much water is coming out of the tank when he empties.
The OP was unfortunately not clear at all as to how he determined his 40 gal tank is only holding 20 gals but if I had to guess it would be that he's relying on that silly tank monitor which as anyone who has been at this for awhile knows will almost always read tank content inaccurately. This is all speculation for now though,
until the OP clarifies. ;)