I see there is a tree at ancestry.com that shows Theresia's maiden name as Billmeyer.
They show Bartholomeus Heiss born Aug. 18, 1812 Aufhausen, Aichach-Friedberg, Bayern Germany died Jan. 28, 1869 Oshkosh, Winnebago, WI
They show Therresa Billmeyer born1910 Wallkofen, Bayern, Germany died May 24, 1878 Oshkosh Winnebago, WI
they only list one child - Ida Heiss born about 1859 WI - died 1944 Oshkosh, Winnebago, WI- married to Gustave Behncke
Ida is in 1870 census , but not in 1860 so she was probably born 1860, not 1859
they do not show where they found maiden name for Therresa
Here is an obit they have for Therresa
HEISS -- In this city, on the morning of May 24th, Mrs. Terresa Heiss, aged 68 years.
With many friends who have given her their sympathy, her life has gone out gently and surrounded by all the tender care that love and affection could bestow. The funeral was held Sunday, and largely attended by mourning friends and relatives.
The Oshkosh Daily Northwestern Monday, May 27, 1878
So I can see no source to show Theresia as maiden name Billmeyer but if you have access to ancestry. com and look for that tree you could contact the owner of the tree and ask where they found her maiden name.
I am wondering if possibly it shows on her death cert. ?? I see the marriage you found for Joseph and that seems like a source at least.