Steep grade, I can't get my truck to go any faster than 50

Ok so I need help, I'm a new driver only on the road a month. I've been driving a concrete tanker. The problem I have is there's a downgrade on I66 it not a steep one at all it's just really long. All other trucks have no problems here but my truck continually slows down. The speed limit is 70mph but I can't get my truck to go any faster then 50. I'm pretty sure it has to do with the load shifting but I don't know if this is normal or I'm doing something wrong. Any help would be appreciated.

 

Okay sorry I should have been a more specific. I drive a Volo D13 10 speed, almost fully loaded, at 79K. It takes the pedal to the floor just to maintain 50mph. It only last for like 3 or 4 miles or so and then everything is fine I just can't figure out why at this spot I am having such a hard time, there are definitely steeper downgrades on my run then this.

 

Its a combination vehicle. I pick up the concrete powder.

 

Sorry I should have been more specfic, I drive a Volo D13 (I believe thats what model it is) 10 speed. I run almost fully loaded at 79k. It is a combnation vehicle, I pick up the concrete powder. It takes the pedal to the floor just to maintain 50mph.