You really need to be able to comfortably run at 55 especially on the interstates which often include long 6 per cent grades. Driving 40 on some sections of the interstate is illegal as the minimum speed limit can easily be posted at 45 mph.
The 6.0 Chevy and certainly the 6.8L V-10 Ford have more than enough power if the Class C and its TOAD connection are properly matched. A lot of folks seem to fear letting the V-10 do what it does best, rev. They make their max HP at 4,250 rpm and you can run them at 5,000 all day long and the only damage will be to your wallet as they get thirsty.
I cannot imagine going down the interstate at 40 to 50 as that is simply a disaster waiting to happen but with either the 6.0 or the 6.8L this should never happen unless you have waaaay too much TOAD {and it does sound like that is the case}. Time to start looking for explanations, methinks there are mechanical issues that desperately need addressing and correction.
As always.... Opinions and YMMV.
:C