We're going to need more info than you provided. What brand/model of furnace do you have? I know some Argosies have Suburban (that's what was in mine) and some have a different brand which I can't remember the name off the top of my head.
I would imagine the valve is available, you just need to track down the part number and then do your typical google search!
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)