Let me lend some comparison statistics and some questions.
1. Our coach weighs 42,500 lbs or so and towing the Chevy Avalanche makes its overall travelling weight about 48,500 lbs. Do you know your weights for comparison?
2. Our coach has a Cat C-12, 505 hp engine, and gets from 4.5 to 5.5 mpg at cruising speeds from 60 to 70 mph. We have seen as low as 4 mpg and as high as 6.3 mpg, averaging 5 to 5.5 mpg.
3. Our coach has a 10k lbs rated hitch. Your stacker weighs 14.5k lbs. Does your hitch safely support that much weight?
4. A friend with a 2004 Country Coach Magna weighs about 48k lbs without his trailer. It has a Cat C-12, 505 hp engine and gets 6 mpg. With a diesel additive - Lynch Diesel Fuel Treatment, $9 per bottle per 100 gallons of diesel - he claims to get 7 mpg.
5. Our coach has an AFE oil-bath type air cleaner. I think it should be cleaned more often than I clean it, but cleaning it takes a couple hours total with overnight to dry. The detergent and oil cost $30 per cleaning. On the other hand I don't have to purchase expensive air filters. Cleaning the filter more often might yield better mileage.
6. I run our tags at 110 psi and the tag push-down pressure at about 30 lbs. I reduced the push-down pressure from 35 to reduce the weight on the steering axle.
Four and one-half years ago when we were first looking at motorhomes, the first coach we saw was a 2006 Patriot Thunder homed right here in Hermiston where we live. He was asking $165k and it had been for sale for over a year. He told me his coach had a C-13 and he got about 5.5 mpg. It was a beautiful coach and we should have bought it right then and there! But I was afraid of the low mileage. I guess mileage wasn't such a big deal, eh? Certainly, diesel prices are not as bad today as four years ago!
Just my opinion... based on my experience with our coach, I think your mileage could be improved some but probably not much more than 1 mpg. But at 3 to 4 mpg, even a 1 mpg improvement is significant.