Setting aside cancer dog, 95% of the time when I see this lettering on a vehicle I know it's probably mental illness.

The other 5% is probably the best plumber in town

Entirely seriously:

I once dated a person with diagnosed schizophrenia.

Obviously I cannot diagnose someone from their truck alone, but my first thought here is that this person is convinced their dog is being fucked with by their 'handlers' or 'gang stalkers', who have magic nonsense technology and tens of millions of dollars dedicated to fuck with him and his dog in particular, capable of using some kind of DEW to project thoughts and memories and voices into their had, beam cancer at their dog, etc.

... Or, that is my own CPTSD from having had a fairly longterm relationship with a schizophrenic talking.