The Suburban NT furnace should have a gas valve outside of the metal cabinet under the sink. On the back wall near the bathroom left side there should be a thermometer that allows you to set the temp that triggers the furnace. By sliding the temp tp the left the furnace should click and start the blower and trigger the ignition. You might want to get a CO2 leak detector (Home Depot has them at the front of their stores now on special 2 pack. Watch for rusted components and dead critters/ wasps that love to make nests in old furnaces.
Look under the belly pan. There may be a valve in the gas line going to the furnace.
I had a similar unit, 1973, 25 foot....I was informed by the Airstream Dealership in Greensboro that the furnace had been recalled but that the unit is so old that the recall is no longer effective,,,,,,and was advised not to use the unit due to a possibility of gas fumes leaking...
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