So how would you identify a reversed hot and nuetral before you electrify your trailer? If there is a Double Pole Breaker and an Indicator Light, you would know as soon as you plug in and before the trailer gets electricity.
You do as you wish, for the $35.00 or so, I will use a double pole breaker.
Actually here is a link to West Marine, scroll down the page to #335524. This is a panel for $77.99 with a double pole 30 amp breaker and polarity lights.