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Truck fuel economy is pretty well cast-in-stone. Still, the the difference between drivers of otherwise identical rigs (big trucks) is 30%. That's a huge spread. Some drivers just get in and go. Others (the winners) know the details of the day in advance.
Trip Plan is the truck industry name for best utilization of equipment in re time & distance. It involves the use of different analytical tools. Most parameters are absolutes (staying on pavement, obeying traffic laws), so the margins are exploited to gain small increases that accrue over time.
Were I using a motorhome (and I have, going back over 30-years) my first thought is that the shortest engine-on time is a goal to post. Second is the distance to be covered (where the origination and destination are both known). Third is weather. Fourth is daylight hours available.
Within these are the items which can affect total mpg: best roads (even if a bit longer), estimated fuel burn (where do I re-fuel; consequently, where/when do I eat & rest); what major metro areas must I traverse (they tend to be at least 100 miles wide on any road), what temps, winds, conditions will I encounter? Altitude changes? Etc.
One needs to know the point-to-point distances to correctly estimate driving time on a daily basis. 50 mph for all hours is a standard, to include moving or stopped. But, as you are NOT on a schedule in the same manner, a different numbers may work, but you MUST keep a log of how things go in order to isolate and improve.
In other words, before the key turns one must know not only routing, but EVERY stop beforehand; right down to how to enter and exit a particular location (GOOGLE Street View). Some choices are far better than others, as all truckstops or rest areas may not be open, or be unpaved, or exist after three turns and three controlled intersections away from the highway, etc.
Motorhomes are notorious for terrible steering (sloppy, huge dead-center) so that is where I would expend any funds prior to aero aids. Second is brakes. Then FE alignment. Depending on year model, ignition, exhaust and fuel delivery if gasoline. A close eye on hub & tire temps/pressures, etc.
The fewest stops/starts is key. The smoothness of each is secondary, yet additive.
Within this, aero starts to work. Unlike a car one cannot just follow the front wheels around and keep gains consistent. The percentage difference of a 1/4-mpg on a moho are huge (and practically meaningless for a car), and easily, easily lost.
The trip plan means all contingencies have been addressed (takes experience, too). If high winds are expected in the afternoon, one might be advised to avoid same by a late, long, lunch and an early stop to the day (campground), for example.
The "aero" motorhomes are low to the ground in their class: VIXEN, FMC, GMC. Note mirrors (big deal) and other items hanging off body. Chin spoilers seem to have been attached to some Class C types since the 1970's.
Modern motorhomes (SPRINTER-based) rely on excellent drivetrains/gearing to achieve high mpg. A boat-tail on one of those would likely be the "capacity" (motorhome abilities) versus fuel burn champ.
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