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.
Location: Just North of Detroit, a surprizingly great city
PIExpress: why do you want to know fuel consumption at idle? As Phil said, and I've been told by many DD mechanics, this particular engine tolerate long periods of idle. I have not asked nor received an explanation as to why it is worse for this style of engine versus 4 stroke cycle engines. I've just never needed to have long periods of idle with my coach. Having said all that years ago I drove a couple highway buses that used a high idle setting (8-900 rpm) for idling periods while the bus warmed or cooled in preparation for passengers.
Jon and Alie Kabbe
Started with 77 Coach
Now have 39' 93 coach
2007 civic toad