Paging David (turbokinetic)!
We've had a few refrigeration issues posted here recently and David is our resident expert/miracle worker. Unfortunately, he's busy and out on the road a lot, and hasn't been posting much here recently.
That you even got anyone to come and take a look at your freezer is a real accomplishment. If the mechanical components of the sealed system are functioning properly, then it could very well be that a leak has developed over the years. A re-charge with original or even a reasonable substitute refrigerant could be expensive and would likely provide only temporary resolution. Locating the leak could be difficult and would run up labor charges trying to find it, but if it's worth it to you to have this freezer operational again, then would be the way to go.
I don't know where Marengo is, but someone else who reported some refrigerator troubles has family members with knowledge on mechanical systems. If by some chance you happen to be within reasonably close range, they could be a resource for you.
