Do your staff charge up the batteries overnight every night?
Do you have a b+b charger installed in the vans?
Do they fire up the heater with the engine running first thing?and keep it running with no shut down cycles throughout their working day?
I use 2 x 105ah numax batteries and they last around 3 years before needing replacing....
I bet your staff are draining the batteries too low, too often because their not doing the above things day in,day out.....