That 8 sounds low, I'm guessing due to the terrain. Also, wind can make a huge difference. I averaged 10 mpg with a half ton Chevy pulling a similiar size trailer. Some days 8, some 11 but the average over a long trip was 10 and your's might be too. 1000 miles isn't really a very long trip to get a true average. One point to consider is that it will be very difficult to make up for the cost of trading trucks with better MPG especially considering diesel runs from 20 to 90 cents a gallon more. Yours is already depreciating but you can stretch that out over a period of years. Buying a new one will start up the cycle of depreciation again. Diesels cost more in other ways also. There's much more to the cost of RVing than the cost of fuel. But the diesel is guaranteed to pull stronger.