Normal for onan 12.5kw to die when coming off shore power?

I think it's more akin to starting your car on a cold morning, flooring the accelerator and slamming it into gear. I am surprised you haven't seriously damaged anything.

This has been debated before but I always go under the practice of unplugging before starting the genset. But yes you need to let them come up warm before throwing an AC on, let alone 3 at the same time.