I hope you are doing it anyway, for the protection of the tires, but you will want to make sure that the tire pressure is at it's optimum level, as this will help with mileage too.
There is a PSA (Public Service Announcement) here in the states, that talks about how you can safe enough in gas to buy breakfast for a year. I don't remember all the suggestions but here are a few. (Maybe some other folks will remember the others)
1.) Proper air pressure in the tires.
2.) Have the engine tuned up, working at it's most efficient.
2.a.) New Air Filter.
3.) Clean the vehicle (every little piece of dirt will have some effect on the aerodynamics.)
4.) Drive slower, at the engine's most economical speed.
5.) Don't do Jack-Rabbit starts.....and coast to a stop by watching the road, traffic, and signals up a head.
6.) Reduce the weight.....gonna be tough in an RV, but the less you have to carry, the more you'll save in fuel.
Granted, all these things at the maximum efficiency, may save you all of 1/2 mpg, but for your trip, that may be quite a few bucks.
Bottom line, most RVs seem to be in the 6-10 mpg range.....you won't find something that will suddenly give you 20 mpg, but you may get lucky, and get it up 1 mpg.
As someone mentioned....pray to the RV Gods, for a tailwind!:)
Not much else you can do.
Have a great trip!