If there is a small black wire connected direct to the battery's + post, it might well be the hot wire out to the breakaway switch.
Can you post a pic of the thing you think might be a contactor? It is always easier to identify things from pics.
But taking a guess here, if the part you are referring to looks like this:
Then it is a circuit breaker and is necessary in the line. Replacements are readily available at auto parts stores for a lot less than you would pay at an RV parts counter.
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'11 Cougar 326MKS loaded with mods
'12 Ford F250 SuperCab 6.7 PowerStroke Diesel
Amateur Radio: KD2IAT (146.520)
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