Disconnect the fuel line going to the pump first and see if you get fuel there. If fuel does not run out the line, then draw a vacuum on the line and see if you can get fuel to run freely. You can use a longer piece of clear tubing and draw a vacuum with a hand vac or your mouth until you see fuel in the line. Drop the line low and into a jar. Verify that the fuel runs freely with a natural siphon. Once flowing there shouldn’t be any air in the fuel. Keep drawing fuel until you get fresh fuel out. If you can't get fuel out, then it may be an obstruction in the fuel line or a problem with the dip tube in the fuel tank. Once you have fuel coming out of the fuel line, then reconnect the line and see if you get fuel with the pump. If no fuel then disconnect the fuel filter and see if you can get fuel that way. If no fuel then the pump is shot.
Be careful of getting the cheapest pump from an auto parts dealer. Although the pump does not have to provide a large volume of fuel it does need to be the correct pressure. Too much and it will over whelm the float. Too little and you'll run lean under load. The rated pressure should be around 4-6psi. Many have had good success using the automotive pumps. Carter makes one that is really a good quality pump that looks OEM to Onan’s pump but more like $80 v/s Onan’s $225. You could also go with a Mr. Gasket vibrator pump for around $40 but be prepared to replace it more often.