We have a 2012 24' Class C with a 2011 E-350 6.8L V-10. Per the Ford website max HP is 305 at 4,250 and max torque is 420 at 3,250. You can run 4 - 5,000 rpm all day and (other than burning a fair amount of fuel), you won't hurt a thing.
Most 6% grades are easily handled in 4th gear at 3,200 to get to the top at 55 - 60 mph. An occasional downshift to third (passing on a hill), will generate around 4,200 but it doesn't last long. Can't imagine anyone needing more than 4,200 very often or long.
:C