I agree with Karla, would be interested to see what the other blood
work shows. Tick borne disease is a possibility and they can run blood
tests for those but, at this stage, maybe trying the Doxy would be worth a shot, but then I'd be worrying about kidney/liver function.
Did you even ask your Vet to call one of the University clinics for a consult?
Feel bad for you. Never easy decisions.
hugs and keep us posted.