Hi. Welcome and congratulations.
I found this "spec sheet" for 1973 Argosy trailers on the Airstream website. I didn't look hard but didn't see a 1974 there...
Do you know how long it has been sitting? You may need more than a spare tire - you may need 4 tires and possibly new axles and brakes just to tow it home safely.
People here with more experience can probably give you more specifics but please be cautious about this and be sure it's roadworthy.