I have a 1998 Ford E350 Goshen Bus.
It has a V8 5.4l SOHC engine. I drove it for about 50 miles and it stalled. Speed decreased so I pulled into a parking lot and shut it off. I couldn't get it started again. I thought it was a fuel pump fuse, it wasn't. Then I thought it was the fuel pump, I replaced it (I was going to do it anyway) and it still didn't start.

I'm not sure what the problem is now. Please help.

 

This line of reasoning could be wrong, but since it lost power kinda gradually it sounds like it ran out of fuel.
Do you hear the fuel pump run for the usual 2 seconds when you turn on the key without trying to start the vehicle? Need to check for fuel and spark. Remove a coil and place a screwdriver in the end and with the screwdriver close(1/4) in to an engine ground , see if you have any spark when cranking over the engine. Need to know what you have or what is missing.
Did you just purchase fuel a short time ago?