Hi, Scott
What they are saying s true :applause:, you should be able to keep you Gen set running till you are down to a 1/4 tank.
At this point it is hard to say why it stops at 2/3 of full... but something is not right. :indecisiveness:
One more thing are you sure it stops running from the lack of fuel, and not from some other reason
Most Gen Sets will shut down for "Low Lubrication Oil", "High Temp." & other reasons, ....before you do anything make sure you know "Why" it is shutting down.
Is you fuel filter clean?, sometimes with a full tank you get sufficient flow, and then as the level drops you get less flow, the same would be true... for a small air leak on the suction side of the fuel line
I don't know about your RV, but most fuel tanks will have to be removed to be worked on, as the fitting are on the top of the tank...
good luck