You measured the tank and it seems like everything checks out. What if the tank is plumbed incorrectly and the generator draws fuel only from the top half, or is there something inside that tank taking up space. Another possibility is that the fill cap does not vent properly and you get vacuum buildup inside the tank.
Edit: Could you have a double wall tank, that would explain what's going on.
2013 RAM 3500 Dually Longhorn
2014 Trilogy 3650RL, W/D, Onan 5500, 8000# Axles, IS, 960 Watt Solar, 6x6 Volt Battery Bank, Freedom SW 3012 Inv/Charger