I agree with westend that if you can get it for the right price it's repairable. If the appliances work, the rest of the repairs are more time consuming than costly. Go through the trailer and use any issues to beat them down on the price. Push on the outside walls to see if there is any give (rotted studs) and push on the ceiling. My guess is there's a good deal of rot with the roof being that neglected, but there's really nothing in a trailer that can't be fixed.
I'd assume, based on the lack of maintenance it also needs brakes and bearings in addition to tires, but they're fairly cheap if you plan to do the work. This is how we got into our trailer and it was a good (and sometimes frustrating) project. If you're looking for a project go for it.