Do you mean plugged in on the LEFT? I see nothing but insulation on the right.
If so, that is 120VAC, and is only powered when you are connected to shore power. Fuses do not have an effect on 120VAC power. Circuit breakers (the flip switches near the fuses) control the 120VAC current.
Your furnace is entirely 12V and operates off battery power.
Lift the front panel off your furnace, and there should be a small switch on the front. For now, you can just turn it off there and put the fuse back in so your fridge works.
The furnace cannot be accessed from the outside. It must be pulled out of the cabinet to be serviced. You may want to have a professional tackle it if you want the furnace to be functional. Otherwise you can just leave it since you don't use it anyway.