It is a support vehicle, attached to SOT. One is a Merc Sprinter base vehicle with box thing on it for additional rescue gear etc and the truck has more gear, supplies, hazmat stuff (CBR), the works.
Not seen it, but they are there for large scale incidents and multi casualty incidents. Think the truck also has room for a command area.
SAAS does have a comms/command bus too for forward command etc.