After more than 60 years in the business, this is very depressing.
According to this Journal Star article:
"The restaurant chain opened in 1938, and at its height, there were 90 locations from Chicago to St. Louis, Thomas said. The Peoria-area Velvet Freezes added Wonder dogs to their menus in the 1950s."
I'm glad we visited in December so my husband could have one last Wonder dog fix.