What could lead to speed derated? ('07 WS 4900FA BENZ DIESEL)

I was driving back from KY today with a load of Dry Wall and noticed towards the last hour of the haul that the rig was struggling to make top speed (limiter is set to 65mph) .. it would take it a few minutes to crawl up to 63-64mph on flat roads, and then the slightest uphill would quickly slow me down to 55 or so.. I also noticed grabbing higher gears, rpm would be hesitant to go up, though it would eventually hit 15k rpm, it would fight for a second or two before hitting that mark.

If you're mechanically savvy, could you list the number of issues that could cause the truck to behave this way? Its been a rainy past couple days.. Could water be the culprit?

 

Am going to change the fuel filter tomorrow and see how it performs after. Brakes I just did a couple weeks ago so can't be that.

 

That would definitely not be good

 

No sir it is not

 

I'd say so, its a 2007 model pre-emissions