My manual says the same thing although I've never tried it. In thinking about it, the pump has an inlet [from the fresh tank] and an outlet [to the trailer interior]. If the pump could "fill' the fresh tank from the city water inlet, seems like it would have to pump backwards through itself.
2012 Eagle 320 RLDS
2012 Ford F-250 Crew Cab FX4, 6.7l PSD
3.55 E-locker, Timbrens