NG has a lower calorific value than either LPG or petrol so it will take more of it to get the same bang. I didn't notice any comments about range per tank, so I would expect it to be less than LPG wich is less than petrol.
Dual fuel does not typically work well with any vehicle because the engines are designed to run on a particuler fuel. In order to get the most out of NG's higher octane level you would need to increase the compression of your engine. This could be done with timing but that is not as effective as changing the compression of the engine. I'm not sure what the optimal compresion would be for NG but LPG likes between 10-12:1 so NG should be higher. I don't know how you could have the engine change that much at the flick of a switch. Instead it would be set up to run OK on petrol but be gutless on NG. This is how petrol/LPG dual systems are.
I haven't looked into it for awhile so maybe things have changed. Look into how dual fuel petrol/LPG vehicles run and then assume NG will be worse because of the lower calorific value.
