The injection pump is fine or it would not run properly on a container. The problem sounds minor just finding it may take some time.

Air leakage into the system before the lift pump or fuel not available to it.

With just five gallons in the fuel tank it may not be a high enough fuel level in the tank to feed from your return line. You need about 1/2 a tank to submerge the open end of the return line in the fuel tank.

Do not give up as you are partway there.