The kits you see on here have all the necessary items to re-use you existing shore power cable. I have never done one yet but looks easy enough. I have heard sometimes the hole is larger than the kit provided back plate and one has to install a larger back plate first.
I will be doing one to my OFF-ROAD POPUP trailer this coming season to go along with my planned Battery and Inverter upgrades. I want to run my Inverter 120VAC output to a connector right next to the shore power cable connector and then just use a RV 30A -20A adapter cable to hook my between my Inverter and the shore power cable receptacle. Pour man's transfer switch hehe...
Always working on PLAN B things here
Looks like a good project for you change out your shore power cable. Think the kit is around $100