24V truck battery - voltage when the engine is running?
I am familiar with buses (especially Mercedes Benz) -they are 24V-vehicles as well. Anyway, most of buses I've seen have around 28VDC (Actual value is between 27.6 ~ 27.8V) while the engine is running. Shouldn't be different from trucks.

PS: ISO-16949 and ISO-16750-2 suggest that electrical tests should be performed with 28V supply for 24V-products (and with 14V supply for 12V-products) since the battery voltage is 28VDC while the engine is running.