Price sounds reasonable for what you're describing, but on that vintage are you sure you don't need to replace axle(s)? If they're not solid axles but Dexters or similar, they may be shot or near it. New axles come with brakes and hubs. I doubt I'd spend money for a fifth tire. Our 87 Sovereign
has no place for a spare anyhow. We have a roadside assistance plan, and I'm told you can limp short distances slowly on 3 tires if necessary. If I did have a spare, it would be a used one, figuring it would have to be replaced in about 5 years even if it had never been on the ground.
My 2 cents as a relatively new guy -- you'll get lots of opinions and no doubt some will be better than mine. I'd ask a knowledgeable Airstreamer in your area to have a look-see.