In addition to all of the great info Carolyn gathered. I checked the old newspapers at ancestry and happened upon one that will keep you busy for awhile
Daily Northwestern (Oshkosh) 31 July 1933 HEISS Reunion at Pine Lake, near Slingo *(the following is the short version:) 80th birthday of Mrs. Whilhelmina (Heiss) Fuhrman, daughter of Bartolomasius(?) Heiss, founder of American branch, came to US in 1851. A few years in Milwaukee, then to Oshkosh area of Winnebago County. He was the father of 17!! children.
Local attendees (rewriting surnames as newpaper printed, so they could be off a bit, image not too clear)
from other cities - Fuhrman, Simerson, Magerfleisch, Berg, Herbst, Paxton, Taylor, Henderson, Schafer, O'Connor.
One of the earliest obits listing survivors was 1909 When William Heiss died. wife and 3 sons.3 brothers, Andrew, Joseph, and Benjamin of Oshkosh. sisters - Mrs Apel, Mrs Schneider, Mrs Behnke of Oshkosh and Mrs Fuhrman and Mrs Victoria Wendel of Milwaukee.