Transmission pictures don't show the valves , but do have a picture of the heater box on the firewall , under hood above the gen set.
The X marks the heater hoses, look at all the hose you can see for a manual shut off valve.
The circle is around the hot water flow control valve , this one is electronic , yours may be too, if so make sure the wiring is properly connected as someone may have set the controls to max A/C then disconnected the wiring to stop the water flow.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.