I do think it very unlikely, however.... but yes, it's possible.
First - like you said - I very much doubt a Rattlesnake bite would be fatal to a dog as big as Rocco... who must have weighed 120 or so?
Secondly - I can't remember the last rattlesnake I saw in this area.... decades ago. The last I heard of anyone seeing one was maybe 25 years ago and about 3 miles from my home.
BUT.... due to no water and thousands of acres of farmland going out of production I have to think that some animal species could be moving in, or nearby. Maybe Rattlesnakes are returning. I know they still inhabit the Mendota Wildlife Refuge and that's only 2.5 miles from my home... but again, I have a lot of land between my home and the Wildlife Refuge and if rattlesnakes were returning surely someone would have seen one.
But good thinking... that didn't occur to me. Very unlikely I think but right now something 'unlikely' is probably most likely. - JamesJM