I'm surprised it lasted that long.
As said above... WOT takes a lot of fuel... so if you were playing for 6hr straight, then there must have been a lot of low throttle time.
And yes, the injected Doo's burn about 15% less fuel.
Last thing... your dad's ski... if it's an RFI, it's also got a smaller engine, if it's a DI, then it's the same engine, with better fuel economy.
The quoted fuel at WOT is 14.5 gal/hr for your RX... 11 gal/hr for the RFI, and 12.5 gal/hr for the DI.