(Sad Question) Has Anyone Experienced Something Like This?

I am so sorry for your loss. (I've worked in the Veterinary Field for over 20 years) From the information that you provided, It sounds more towards your dog having cancer vs something poisonous. The symptoms your pet was showing would present differently had it be poison. The blood values would also be off differently. For example your dogs blood sugar level could be lowered, or his blood clotting time would be off. This would be just a couple of examples of values on their blood tests that would lead the Vet to lean more towards poison as oppose to Cancer. Cancer is very common in dogs. From the sounds of it, your dog probably already had cancer that went undetected, and by the time that you first noticed the symptoms it was too late. (Not of any fault of yours or the Vets.)
If you want to know 100% certain, the Vet can always do an necropsy. Again, I am truly sorry for your loss of your dog. My thoughts and prayers are with you!

I am very sorry for your loss. I've lost a beloved pet and I know it is painful. My mum told me she lost one of her childhood dogs to a "brain condition". My mum grew up in country side and they took him to who they could take him to, not really a vet, but someone who understood and treated dogs. It is very common for dogs to die due to something in their brain that, I am told, cannot really be prevented or controlled. I don't know how old your dog was, but my mum did indicate that it could hit them at any age. She did say her dogs experienced it over a few days to a week, but it is definitely not uncommon for it to be abrupt. I pray you find strength to deal with this grief.