Yes, with the OP's second post, we "got the rest of the story".
It was not just an amp draw causing the issue, but evidently a wiring issue from breaker box to outlet in the engine compartment (since the problem persists even with the block heater unplugged).
I am not aware of standard 30 amp CG outlets being GFI, so that part puzzles me. Is it the CG breaker that kicks? Breaker for block heater? Other?
And, as standard Preventive Maintenance, I would (with all power off) open and examine the outlet box in the engine room for the block heater. It is likely in an exposed are where water, dirt, mud dobbers, etc could have "modified" it.