Drifting and jumping tractor

OK, so I am getting this information from my driver who is on the road and texting, so not the best line of communication, but here goes.

2017 freightliner argosy, he says that when he goes through a step driveway, or up a step hill the truck struggles and jumps around alot. Also when driving the truck on straight roads it seems to drift around a lot.

I have driven the truck bobtail and I did notice some drifting in the truck, I thought it was front end and it felt like every now and then you would get pushed from side to side, it was very minor but I am guessing with 50 Tonne behind its a lot worse.

I am thinking maybe the air bags are not at the right height? Maybe the levelling valve isn't letting air out which is causing it to run high which is stuffing up all of the alignment?

Am I on the right track? What else should I be looking at? Drover says it is getting worse.

 

I greased 5th wheel today, but it had heaps on it, so I don't think that would be it.

How fast should the height control valve expell the air when decreasing the bags? It seem to be alot slower then my other truck.

I checked the torsion bars and drive shaft for movement, but they all seemed OK. There isn't any abnormal wear on any tires.

Driver said its worse when going up steep driveways.