You are on the right track looking at 25 foot and trying to stay vintage when you take into consideration your tow vehicle of choice.
The problem with doing that is you fall in love with someting that is either close to the limit or over the limit and now it is time for a new tow vehicle to maintain both your and other drivers safety.
Not saying you won't find a trailer that matches up nicely, just that the trailer that chooses you may not be towable by your mountaineer.
That being said, here are some things to consider in your search. Vintage units ARE lighter. They also have fewer do dads to go bad. Most units, even today are simple technolgy, but vintage are even simpler. Biggest drawback to vintage is the lack of a gray water tank and a limited fresh and black tank. 2 days on fresh water (25-30 gallons) and if you don't have sewer hookup you are carring a blue belly portable tank. Also most vintage will only have 1 battery, so power conservation if boondocking is an additonal concern.
Post 74 you have gray, but it is small up until 77-78 then it starts to get where you can go 3 days if you are conservative on the gray tank. The grey is normally the limiter, not the black. 69-73 had issues with the tambour doors, if they have not broken they were either replaced, or it is time for them to break. 66-68 there were these neat framless windows, curved corning class. They can break and are hard to find, or you need to budget for it.
To stay in the size you need to because of the mountineer, you are looking at mostly single axle trailers in the 60's and some dual in the 70's. Names like Safari, Tradwind, Caravanner, Globetrotter are all in the 19-25 foot range. Most 27 footers will be called Overlander and will be dual axle, but they may be over your limit. to see pictures of the units I am talking about look here : http://www.vintageairstream.com/archives/
For weights to help you narrow your search look here: http://www.airstream.com/airstream/p.../weights-1.pdf
You will need Adobe to read it.
This is the time of year to buy one if you find one you like. Winter in your neck of the woods tends to depress prices some. If you want to buy from a GREAT dealer it is worth the drive to Ace Fogdall in Cedar Falls Iowa. I bought 2 trailers from them. Good dealer that delivers a 100% working coach, new or used. 1-800-747-0747
Buying from a dealer will save you from the to-do list syndrome that many of us have when we go private party.
Good luck on your search!