Truck fuel=Plane fuel?
The same fuel from the nitro power R/C truck can be used in the airplane (and the airplane fuel can be used in the truck) It may not give the best results though....
The R/C airplane will generally not need as much oil for cooling, and will not use as much nitro as is normally used in the R/C truck. The engines are designed just a little different (usually the port timing) in order to do thier intended jobs... and the fuel diffference will mean you won't get the best power either.
Using the truck fuel in the airplane will probably cost more than having 2 separate fuel types. The truck/car fuels generally are more expensive than the aircraft sport fuel.