Hi tedxas,
The first question is how much water is coming out of the hose that you use to fill the fresh tank. If 1 gallon/minute, it would take around 50 minutes to fill a 50 gallon fresh tank. And that's assuming the water doesn't back up in the gravity fill and spill out.
See how long it takes to fill a 5 gallon container using the same hose. Multiply by 10 and that'll tell you how long it'll take to fill the tank.
Sometimes the inexpensive water pressure regulators restrict flow as well as pressure. If you're not getting a lot of flow through the hose, try again without the pressure regulator. If you get improvement, you should look for a better pressure regulator.