My 1978 original Armstrong AC is still going fine. The only things I've had trouble with is keeping the filters in place. They would invariably fall out during travel and the filters we got with the unit were pretty minimal--just a flimsy piece of thin foam and they would have a tendancy to get sucked up into the unit and then allow unfiltered air to pass around the sides.
I came up with a pretty BAD looking, but pretty functional solution. One day while I was rescreening windows I looked up at the AC filter and thought, Hmm, what if...
You can see the pics below. I made my own filters using a sandwich of a layer of 3M Filtrete
hypoallergenic filter backed by a layer of carbon filter which I found at Home Depot for use in AC's. I ditched the very old thin layer of foam which you could hold up and look right through. That was a very good filter...
I won't win any awards for looks! But, this setup works really well. It stays in place! The filters catch a tremendous amount of dust/dirt. We had it on all week at the Rally and changed the filters because they were really yicked up. The charcoal does a miraculous job absorbing cooking orders which are otherwise kind of trapped in a closed up trailer.
The combination may cut down on airflow a bit, but the fan didn't seem to have any problem sucking air and it spent the entire week in Missouri heat on HIGH! The screen keeps the filter from being sucked up into the AC shroud which would otherwise happen with any filter media offering much resistance to airflow.
I may make some changes to improve the looks, but this setup functions extremely well! Redneck Ingenuity!