As carringb said, we need info (weights, etc)
It's normal for your C to slow down at 'full load' (numbers?) going up a steep grade. The V10 in your E450 is a great engine and can rev all day long. Not sure of your weights but mine is just under 12k unloaded, and I pull a 3k jeep behind me. So yeah, I slow down too. I've never felt my C on the E450 chassis was a slug. You can't expect to maintain 65 or even 55 on that steep a grade. I'd say 35 to 45 seems about right.
Also, as carringb stated, as long as your maintenance is good, including tire pressures, air filter, oils, etc, it should be fine.
Lastly, the higher you go, the less oxygen (which is why humans tend to breathe harder the higher you go). Same with an engine...less oxygen, less power. Normal. Turbocharged engines (like the ecoboost Ford has) has less issue with elevation because it's forcing more air into the mix.
If you have it looked at, be sure to state exactly the conditions. They will most likely say same thing...normal. It's a huge amount of weight to go up a big mountain.
