Goosey:
RUC needs to stay as is, abeit could possibly be more user friendly.
Reason: Diesel is used for more applications than just automobiles.
Leaving RUC's the way they are long term isn't really an option. When we decide to end the plug in car exemption, without RUC changes, a Nissan leaf or similar electric vehicle will be paying triple the road tax of a Prius. This not logical, and creates an incentive against electric cars (of which we have a government policy to increase the numbers of...)
Regarding your reasoning, Petrol is also used for more than just automobiles. The magnitude does vary, but off road petrol use is far from negliable. MBIE put off road diesel use at 40%. They estimated petrol off road use at 7%, but report notes the inaccuracy of this number as a weakness of there report.
http://www.mbie.govt.nz/info-services/sectors-industries/energy/energy-data-modelling/technical-papers/pdf-library/Liquid-fuel-use.pdf