https://www.gmcrvparts.com/product-p/g72r8415.htm
This shows an arrangement similar to what was fitted on my Rapido. The hot water was teed into the the engine pipework system at the back of the engine with the pipe work routed down the back of the engine and under the MH floor, then rising through the floor to an under seat locker.
The heater control knob which required turning through about 270 degrees was fitted to the dash on the RHS of the steering wheel. It had a steel wire to control the heater valve fitted close to the tee on the pipework.
Suggest looking for the pipework under the bonnet; you may be able to trace the control knob from the valve.