I'm a little confused...you said,"expect the breaker switch for my 12v
stuff kept on tripping"...do you mean the AC breaker that controls the circuit to the AC outlet for the converter?
If so, you can check that AC breaker for integrity, IE is it worn out? Or is there a problem with the AC wires that run from the AC breaker box to the AC receptacle...?
In any event, your 'direct' connection of the converter to shore power through an extension cord won't hurt a thing - you're simply powering the converter from another source...
If I had to bet on it, I'd go with the AC breaker being worn out from age...the contacts can become pitted, worn, heat hardened, etc...those things don't last forever!