Couple thoughts. You don't need the hot water system on an RV in order to make this contraption work. Plenty of 180 water available in the cooling system.
I wonder how much of this gain could be realized by just switching from a Holley carb to a Thermoquad? Thermoquad does the best job atomizing fuel of any of the v8 carbs used by the big 3.
Is better atomization the only effect responsible for the increase in mileage with the water heated intake plenum, or is there something else at play? Did the effect of a longer intake runner impact fuel mileage as well?