Is there a tank switching valve on your truck? When mine failed it acted similar.
Have you tried gently blowing air backwards from the soft line feeding the lift pump to the tank? I've had debris from a failed showerhead partially plug the lines and cause a starvation issue. I see you flushed the lines but I don't know what your method was.
Have you tried running from a Jerry can hooked up directly to the lift pump? That would bypass all the lines and the pickups just to be SURE. It's a free test which is the best kind.
Have you tried gently blowing air backwards from the soft line feeding the lift pump to the tank? I've had debris from a failed showerhead partially plug the lines and cause a starvation issue. I see you flushed the lines but I don't know what your method was.
Have you tried running from a Jerry can hooked up directly to the lift pump? That would bypass all the lines and the pickups just to be SURE. It's a free test which is the best kind.