Lighter and more aerodynamic you can make it, the better off you are. That includes as smooth of sidewalls etc you can get.
Look at what some of the semi's are doing right now, many are gaining upwards of 10 percent with some of the items they add to the base truck, to make the aerodynamics better.
Will this pay off for some of all of us as RV users.....maybe......one needs to figure out how much fuel you save, over a given amount of time, and will the cost make a savings in the miles I drive. I personally would look for a 2-3 maybe a 4 year ROI on the item added. If it does not return all of the $$ spent, and then some. not worth it. Speed is usually the best factor, ie 60-62 in as low of rpm reasonable nets best mpg.
ALso tires and type can add mpg's.
I would suggest checking out articles from some of the medium to heavy duty truck sites, magazines etc to see what is working best, getting the best roi etc. With this in mind, these drivers/trucks are usually doing 70-100K miles a year. So the ROI is way sooner than some RV'rs that might do 10-20K max per year.
Marty