If it keeps gurgling that means it is sucking air.. Now fact it will gurgle for a long time when you first charge the lines but ... .... if it's sucking air it will gurgle forever.
now why would it suck air
The most likely cause is the inlet strainer on the pump. but you'd have checked that when you replaced the pump Still check to insure it is properly sealed and not cracked.
Then inspect the suction line ONE option.. on my RV the fresh tank drain valve is "T" connected to the suction line. did you remember to close it
As others have said some rigs have a 3-way valve called a "Winterizing" valve that changes the suction line to a hose that can suck pink stuff out of a bottle.. If you have such a valve is it properly set.
Next is a pin hole in the suction hose or a mouse hole.
Finally and this one is rare.. My tank has an outlet strainer as well as the inlet strainer on the pump.. it can get "Slimed up" (in theory, never seen it happen) or otherwise clogged (Again never seen it happen).