Hi John , i have a hobby 750 , 2001 model ……assuming you have the same heating system this is how mine works. ……with water switched off on the control panel but with power to the rear switched on turn the gas on and push the switch top right on the wall thermostat up to the 2 symbols for hot water and heating . if you intend to have the blown air heating on whilst driving you must have the Truma Duo control CS ( crash sensor )( approx £150 ) fitted which shuts off the gas in the event of a crash . if you dont have this and you crash then ….Boom !
hope that helps , tony