Karla Finocchio’s slide into homelessness began when she split with her partner of 18 years and temporarily moved in with a cousin. The 55-year-old planned to use her $800-a-month disability check to get an apartment after back surgery. But she soon was sleeping in her old pickup protected by her German Shepherd mix Scrappy, unable to afford housing in Phoenix, where median monthly rents soared 33% during the coronavirus pandemic to over $1,220 for a one-bedroom, according to ApartmentList.com. Finocchio is one face of America’s graying homeless population, a rapidly expanding group of destitu…