Generally, CG's only use a GFI on 15/20 amp (regular house-type outlet).
Would be interested in hearing if others have run into 30 amp CG outlets that are GFI protected.
And, whether it tripped a 30 amp CG breaker because of a GFI or an overload, since you had the block heater cord UNplugged, I would sure check out the outlet in the engineroom into which you plug the block heater. Could be nothing more than moisture or insect nest connecting hot and ground, ground and neutral, etc. Other possibility is a screw through the wire between breaker and that engine room outlet.
5 minutes of work= peace of mind.