In my view, 70mph is too fast if you want good fuel mileage (which is a relative term with mohos). The conventional wisdom is if you exceed 60mph, you really start to lower your gas mileage.
While I have a Class C, it's a 30ft coach, and weighs about 11,500. So it is a bit lighter than yours, and perhaps a bit less cross section to the wind. Still, I would not expect more than a 10% or so difference.
With the V10, I am getting up to 10MPG if I do my best. And that is going 55-60mph.
My engine is just slightly different; 2 valves rather than 3, so my engine is about 50hp less. However, while you may not see 10MPG, I am sure you will see a significant improvement in fuel mileage.
I would recommend an OBD gauge to measure your fuel mileage so that you can find the "sweet spot" for fuel mileage. Something like a ScanGauge2, ScanGaugeE, Garmin EcoRouteHD, etc. They cost around $100~150 depending on which one you buy, but they will pay for themselves quickly if you gain a couple MPG.
I am using one and have found 10MPG. I started out with the Garmin EcoRouteHD (which is displayed by either my GPS or Android smartphone), but then I just changed over to a ScanGauge2 as I wanted a dedicated display (the Garmin had a nicer display, but I could not view the map when displaying fuel mileage).
Also, look at installing AirTabs. I am thinking of doing that next year. One thing I did notice is when I installed my vent covers. I put in the Fantastic brand vent covers, which are probably the largest ones you can buy (the others would not let my vents open fully), I swear I lost 0.5MPG according to the EcoRouteHD!
If true and not just my perception, then AirTabs should help. Before installing them, I could easily get 10.5MPG or slightly better on the highway. But since then, I am struggling to get to 10MPG.
But I suppose that is the other beauty of using a OBD display; it will tell you if anything you did; from changing tire pressure, re-loading the coach, and so on; how it affects your fuel mileage.
Of course, it could all backfire and make you feel depressed. I had fuel flow gauges on my boat, and I struggled to reach 0.8MPG! That was so depressing that we sometimes did not take the boat on trips. One of my boat buddies suggested using the fuel flow gauge as a coffee cup holder (i.e. set your coffee cup on the gauge so that you could not see it).
But on the other hand, since we got rid of the boat and now own a RV, we are ecstatic at getting 10MPG.
The only thing that works on a RV is the owner...