If it's an area susceptible to damage, you may want to use a whip like this to run between the new splice junction box and the junction box that connects to the shore cord. Note that this is set up for 220v with black/red/green, but the wiring is the same and can easily be used for 120v by placing some white tape on either end of the red wire to mark it as the neutral.
Yes, wirenuts are fine as long as the cables are clamped where they enter the junction box and the box cover is installed.
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