The big Truck should easily be able to climb most of the inclines without needing a running start.

But most of the time the max you can get on about a 2% incline is 8kmph.

The inertia should gradually change based on weight of the truck. Fully loaded it should be hard to get moving to speed on an incline.
Unloaded the truck should easily climb an incline in 1st gear.

Either way, the truck should be able to move forward on an low incline.

I spent more than an hour last night fighting slight inclines that should have easily been a 5 min drive.

adding mud ruts would be an interesting change that only the truck could navigate through.

I've attached some IRL back road driving in the truck.