[wingnut60]

You should have a suction line tied to the pump to use for winterizing. Why not stick that line in a bucket of fresh water, shut off the suction line from the tank, and see how it pumps. If it does fine straight from the bucket, then you have a restriction in the tank line. If it doesn't do any better from the bucket, then there is problem on the pressure side. On my trailer, it is easy to take the pump out--if not a real big problem for you, maybe take it out, hook up to 12 volts and set up a little circulation circuit--suck it out and return to same bucket--to see how it does.
I have the 5.7, and had one go bad--but the replacement is doing fine.
But, have had problems with remembering all the valving to change after winterizing to get it back in operation correctly.
I think it is a good pump, best I have ever had in a trailer.
Joe