Don't see how ANY problem with the water pump could move water from your fresh water tank to the black tank. There is no direct connection.
The water pump ONLY pumps water to a drain point somewhere in your coach.
The various drains in your coach lead to the black or gray tanks.
The only way that water gets from your fresh water tank to a drain is through a faucet. Your toilet and your bathroom sink are connected to the black tank and the other faucets drain in to your gray tank. Find the faucet that is leaking and you have found the problem.