This isn't rocket science. From the 2008 Dutchman brochure, you have dual 3500 lb axles on a leaf spring suspension. They just need to make measurements of the existing axle (hub face to hub face and spring center to spring center), then order a new one - it doesn't have to be the same manufacturer. They should be able to move the brakes from the old to the new, and they'll probably need to weld the spring mounts in the right place.
But 99% they're Lippert or Dexter, so they should be able to get one from the same manufacturer. Lippert powder coats their axles, Dexter paints. There are certainly other ways to tell them apart, perhaps looking at the brakes (assuming Dutchmen used Dexter brakes on Dexter axles, etc.). But Dexters seem much more readily available in the aftermarket - Lippert focuses on OEM sales.
I'd question the competence of that shop. "Big truck and trailer" sounds like they may not be knowledgeable about RV and small trailer stuff. That they - don't know how to/can't find a - replacement would make me worry.