FMS Standard Dash Display not reported on SAE J1939 data link
Been working with similar model Volvos, but with the body builder module output
I had a quick look and DD is part of the FMS standard, however volvo doesn't always broadcast all of the values they specify, (the FM/FH seem to have 3 different groupings of what is available with no clear reasoning why)





I can confirm for you that that value should be broadcast on the body builder module output pins provided it is switched on, P1BNY = 2, as well as the other parameters you have mentioned. 

On plug BBM-B1 pin 29 is CAN kigh, pin 30 in CAN low.