Did you actually give those receptacles a good tug to see if the wire is 'stuck'? I would be surprised if there wasn't a fair amount of slack in the wire as they have to have some latitude for fit when doing the panel build. It also might be stapled to a joist. On the last 4 TTs/5rs we have had, all wiring has had more slack then ever needed and it was easy to replace receptacles and switches with shallow old work boxes and Leviton Decora pieces to kinda match the existing that didn't need replacement. I really dislike those usual RV no box wiring devices
If for some reason you can't get wiring slack, solder on pieces of correctly colored insulation and awg wire for length, cover the splices with heat shrink tubing and maybe even add some electrical tape if that helps the comfort factor and do your update. Don't use wire nuts though there are some crimp ons that will work on the usual RV 14awg wiring.