I guess I better disconnect the 2nd factory battery in my Duramax
Diesel
pickumup.

Not a bad idea...LOL!
batteries can explode, I don't deny that. But if it was that great or
common of an issue, then there would be all kinds of govt regulations
checking for paralell and series battery applications to be in "code"

Almost all your diesel Semi-trucks have not only 2 but 3 and even 4
batteries and no "diodes" or "spark arrestors" installed. they are
however, mounted in a battery box which if I rememeber correctly is
Coast Guard mandatory on boats anyhow.