Cover all the basics first. You need someone who can troubleshoot it properly. The pump must have good grounds and no corroded connectors. Voltage drop tests.
I would also say that electric in tank fuel pumps should be treated as a maintenance item. At least thats how I treat them. Like other items on an rv or a car it doesn't matter how few miles it has, parts go bad over time. Never run an electric pump out of fuel and always keep 1/4 tank or more in it.it should pump just fine with the amount that is in it. Adding more will just make it harder to remove the tank. Obviously an empty tank is easiest to remove.