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

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).