The engines fuel use depends on the LOAD the engine is under. High idle , not humping the air cond (40-80 hp), is almost zero load .Figure 1 GPH per hour of idling.
However idling is not good for your engine , tho it may be required for a couple of min to air up , it should never be used instead of a road run. 100mi , 2 hours every month, will keep your engine , and all the rest of the systems happy , and help not loose your road skills.
1983 Newell Coach