RV receptacles, which are the same as used in mobile homes, are not required to be mounted inside any kind of a box. They are called "self contained" devices, or SCD for short. I've seen Slater, Pass & Seymour and Wirecon (Hubbell) SCDs and I'm sure there's others. They have a plastic cover which covers the terminations on the back and there are no exposed 120 volt parts. Can be installed anywhere in an RV as long as there is adequate depth. They can accomodate three romex cables. They are CSA/UL approved, permitted under the NEC and are not inferior or even "junk" as claimed by some out there. There is no reason whatsoever to replace existing SCDs with a residential device and box or to install a residential device & box instead of an SCD. One nice thing about them is that they are quite shallow compared to a residential outlet box and are good for new construction or retrofit. SCD devices are readily available at RV parts vendors and dealers and on ebay.
There is a special tool for punching down the wires (unstripped) onto the metal slots on the rear but you can use a flat blade screwdriver if you are very careful and do it slowly. The technical term is insulation displacement termination.
If you can't do a splice in a run of romex within an SCD, it would need to be in an outlet/junction box complete with cover and must be in an accessible location.
Info. brochure from Wirecon (section A1):
Wirecon SCDsPass & Seymour SCD receptacle:
For 12 volt charging, you could use something like this Sigma charging center:
