I am very sorry that you and your dog is going through this.
Older dogs and even younger ones do not fair well without their people. The refusal to eat (I have seen hundreds of times) is usually due to depression based on separation of their people. This is common in shelters. There are dogs that will survive and those that give up. It is seen many times.
The lack of exercise and one on one contact maybe attributed to the dog's pains. If living on concrete, this also could be a reason for stiff joints and muscles.
Boarding facilities and shelter/pounds also have various viral bacterias. Get a stool sample analysis as the dog could have gotten coccidia or giardia or another parasite.
At this age, perhaps a full blood panel is in order. Hopefully it is nothing serious but only a vet can provide the answers.