Hey ofdtruckie the only appliance that you will need to actually shut off manually is the stove. The stove has a pilot light which is used to ignite the stove top burners.
If you lift the cover and then lift up the stainelss cook top from the front it will expose the shut off valve which is at the right front corner of the stove.
The water heater, furnace and fridge have gas valves which will not allow any gas to flow until the appliance is actually light.
On your model trailer there will be individual shut off valves to each appliance located outside the trailer mounted to the bottom of the belly pan. Most likely they will be siezed or frozen in place unless they have been used regularly.
The original water heater is a Bowen Model, a 10 gallon one. Atwood makes a 10 gallon version with both electronic ignition for the gas and a built in electric heating element. You will need to run 12 VDC to the control switches which will need to be mounted somewhere convienient inside the trailer, like in the bathroom. You will also need to supply the water heater wilth 110 VAC power for the electric element.
LP Gas Water Heaters from Atwood - PPL Motor Homes
Your original furnace is a Suburban model
This is the replacement for it. Once again it is electronic ignition and you will need to supply 12VDC to the furnace. Suburban NT-30SP. The unit is smaller than original so you will need to fabricate a platform for it to sit on. That way you can us ethe original venting holes in the shell of the trailer. You will also need to rework the ducting sysytem to supply hot air throughout the trailer.
Suburban RV Furnace - NT Series on Sale - PPL Motor Homes
For stoves and fridges there are several options to choose from. None of them are exactly the same dimensions as the originals so mods to the cabinets will be required to make them fit.
Linear Mode
