Perhaps its because your nearly half my age? You tell me.
All I'm saying is I find a large HGV much less tiring to drive over a relatively long distance compared to driving a car,whatever the make / model of car too - I found this with all of my cars, from my e38 7,e39 5 etc down to a Peugeot 309 I owned a few years back, the truck is imo the less taxing to drive compared to a car over a similar distance.
I assume its down to the much better field of vision given your sitting so much higher than in a car?
Incidentally, the concentration required to drive a 6 axle artic I would argue is greater than that needed to drive a car, despite this, as I say, I find the latter far easier & am less likely to feel tired in one. Maybe its because its what I do day in, day out.......