You know what you might be able to do is to get your hands on a natural gas flow meter. They work at the low pressures ( inches of water pressure) encountered with fuel gas systems. If you could find a used one, you could monitor the flow directly and be pretty safe about it. But, The OP's country, State, County would have regs concerning the issue and they should be consulted before proceeding. A camera watching a currently installed meter would pretty much be a slam dunk as there would be no modifications to the system at all. The problem here is the resolution would be pretty low and if you were looking for a daily or even weekly flow rate data, it is not going to happen by watching the liquid level in a 500 gallon tank. Your going to have to install some kind of gas phase flow meter.