If you haven't collected them, there were family death notices for Georg Ertl in the local German papers, as well as other members of this Ertl family, like Frank (Jr.). There are about 60 entries for the combined spellings Ertl and Ertel found in "Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950).
Indexing Milwaukee's German newspapers since 2007 ...
Gary Rebholz, compiler/editor
Milwaukee Wis.
gary.rebholz@gmail.com