Beware the Beatrice years..
Welcome! There were no particular years known for lots of leaking, but the years when Airstream owned by Beatrice Foods (during 1970's) there were some cost reduction ideas implemented that led to weaker frames, rear sagging and leaks that led to sub-floor damage.. Some have been restored, some have just been re-carpeted for sale.. Be careful, and consider taking an ice pick or awl to look at any trailer and check for floor rot...
That said, used tralers fall into several categories.. Many written about here are rotten rusty projects acquired almost for free and which become multi-year adventures.. The second category are late model used trailers (3 to 6 years old more or less) that have all modern conveniences and are often in great condition, but still cost more than half the value of new ones. The third category is beautiful restored vintage, from 1950's, 60's and 70's.. New appliances, interiors, plumbing and tanks, and electrical work.. Often pricey, very cool, and come with club of enthusiastic owners.. Finally, there are semi-vintage, from mid-'80's to late 90's.. Often inexpensive (rarely more than $20K) but with fully functional kitchens, A/C, good plumbing and cabinetry (those were years of actual Oak..) and usually tired fabrics, but can be spiffed up without major mechanical work... We bought into latter category, and have been very happy. Other folks here have tasted all 4, and most feel good about choices.
The only major warning is to avoid the rusty major project unless you really have the time, money, tools and skills to get it done. There are not a lot of success stories from people who were unaware of the magnitude of effort when they started, and learned difficult lessons...
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...