I suggest that it would either be dodgy battery leads/battery or connections. Or it’s just worn out. The solenoid usually just won’t throw the starter motor into gear and just clicks.
I had a Kenworth with a 14 litre Cummins that had 4 N70 batteries trying to drive the starter motor. It was always a slow cranked especially in winter. In the end I could get it to start and got the auto sparky to look at it. Turned out it had a braided earth lead between the engine and chassis. It broke. But when he replaced it with a new insulated earth lead the cranking speed went back to what it should have been.