03 Gulf Stream Class C E450 Super Duty Slug when climbing...

On a 6% grade, the transmission should have been in 2nd gear, and at 45 MPH should be able to gain speed.

But... If something slowed you down to 35 MPH, that's too fast for 1st gear, and 2nd gear isn't making enough surplus power at that low of an RPM to really accelerate much. That's really the achilles of the old 4-speeds.... The gap was too great between 1st and 2nd, so Ford fixed that with the 5R110 in 2005.

Now, if you were already going normal highway speeds, and your speed dropped that much... I'd consider that excessive under normal conditions. Do be aware that the PCM will reduce power if the transmission is too hot, so that could have been a factor as well, but you can only monitor trans temps with an OBD device or aftermarket gauge.

When you say that the engine "chugged", does that mean you didn't let it downshift and rev up?

45 or 50 sounds roughly right to me for that kind of a grade, based on my '98 with a V10. That's with the go pedal on the floorboards and the transmission likely in third gear (out of four--I think you also have the four speed transmission and the five speed was introduced a year or two later).

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.

Also at about 4000 elevation. Father in law said she was great going over the cont. divide so I'm currious why a 6% grade would slow her down.

All maintenance up to date, nothing in her other than our clothes and a cooler, 6% grade and she chugged at 35-45mph and wouldn't go any faster. I've heard this is normal just wondering if we need to have it looked at?

What is "being a slug"? I assume you mean it didn't perform to expectations, but what exactly did it do or not do?

What was your total weight, elevation, and grade steepness you were attempting?

Preventative Maintenance all up to schedule?