8-10 is about all anyone can expect with that combination of truck and trailer.
Here's how I have always looked at gas mileage.... The fun, family time and relaxation I get from being out in the RV especially when it's in some forest, near a lake or river far outweighs the cost of the fuel used.
I no longer worry about what mileage I get. I worry instead about the FPM I get (Fun Per Mile) Or (FPG= Fun Per Gallon).
By the way on level road or gentle rolling hills I use the cruise control and overdrive. It's one less thing to worry about.