The OP has a chance here to insert his own Surge Guard and use lots of wire nuts ( and drive the electricians here nuts too)
IMO wire nuts would be ok for this job. I did that with the shore cord on the 5er we had and the portable Surge Guard. I moved the Surge Guard to be permanently inside.
The shore cord's bitter end was to a junction box before going to the power centre. I undid the shore cord from the junction box. I cut the ends off the Surge Guard and aiming it the right way towards the pedestal end, joined three wires to the junction box.
Now I had two plug ends to play with and the bitter end of the shore cord to link up. I forget why, but I used the two Surge Guard plugs and got it all together using lots of wire nuts. Worked great for over 10 years.
For selling the 5er, I took that out and put the shore cord back where it was OEM. Put the Surge Guard back together with lots of wire nuts :) and now it is inside the Class C still working. ( I use the C's OEM shore cord all inside to the Surge Guard and another cable for the RV to pedestal run. I can also plug into the inverter instead of into the cable that goes to the pedestal for Whole House)
I used mostly those big blue wire nuts because they have the side wings so you can twist harder on them. I didn't know the red ones with no side wings were for 10 AWG as was mentioned. I use red ones for # 8 but find the blue ones can be twisted tighter even with #8. Whatever works! :)