you probably won't find any parts for the old suburban furnace; replacement is likely your only option, there.
as for the catalytic, it doesn't give off CO; (thats what the "catalyst" does. its not really "burning" the way an open flame does). it gives off CO2, and consumes O2, so you should probably leave a window or a vent cracked. Even so, the Airstream isn't that air-tight that it should be a problem.
as for "why not use an electric heater instead?"...that's the whole point of the thing...it doesn't need any electricity, so you can use it when there isn't any available.
The furnace is handy, but it does consume an enormous amount of electricity...while it will run off the 12v
system, it can drain your batteries overnight.
I wouldn't run either without a functioning CO detector in the trailer.