Undisolved Vitamin pills in a port-a-potty--Urban Myth?

I've seen in ads for vitamin supliments the claim that most vitamin pills p right through you and end up in the toilet undisolved and then they claim that their "Uncle" cleans port-a-potties for a living and finds a ton of undisolved pills in the drain. Is this an Urban Myth concocted just to sell expensive suppliments? Even if it were true that they p through you, wouldn't all the uric acid and the chemicals they use in there disolve the pills long before the cleaning guy got there? Anybody know someone personally who owns a port-a-potty business to verify or deny this claim?