Welcome to the forums... do a search (Google may work best) for "69 Safari - wiring diagram" could have some good info for your trailer. '69 is a first year lots of new things trailer and doesn't much look like my '68. Look through all the outside panels, there are 120 and 12v
systems and could be more than 1 fuse panel. If you find a blue "uni-volt" there could be a fuse panel in the end. Regardless of all that, if the thermostat is all the way off heat should not be called for and the furnace should not run. Or find the furnace and unplug the wires.
Good luck, Mark D