It comes down to simple math. Bigben's 4 MPG guess isn't unrealistic at all. If anything it's too high for large motorhomes going up steep grades.
Any gasoline engine will need to burn approximately 20 gallons per hour of fuel to make 300 HP; more or less -- engines vary just a little.
So whether you have a V10, Hemi, 5.0 V8 or EcoBoost for that matter, if your rig is large enough to only go uphill at say 40 MPH while using 300 HP, you'll be getting +/- 2 MPG.
(40 miles/hour) / (20 gallons/hour) = 2 miles/gallon.
I'm sure than on really steep hills like we have in Texas, some large rigs may only get about 1 MPG while climbing at 20 MPH or so.