When I Googled it I got to the GeoServer 2.12.x documentation that suggested I use an addition to my query
SortBy=&. So here is what my URL looks like:
So that didn't work, no error so I assume that GeoServer ignored the SortBy.
EDIT: OK, I'm an idiot! It was working all along, I just didn't recognize it. I was judging whether it sorted correctly by the display order of the markers in Leaflet. As user30184 pointed out, without including the SortBy field(s) in the output it's hard to ascertain if the sort worked correctly. I did some experimentation with adding +Desc to the sort field to get it to sort backwards and did some key comparisons and see that all's well. Adding credit because user30184 helped with correct syntax, plus I learned a bit from the answer.