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

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?