Install additional 110 volt outlet in RV

Forget about using a residential type outlet box and receptacle. What you want is a "self-contained device" or SCD for short which are used in RVs and mobile homes. No box is required and can be installed in an existing wall. There is a plastic cover on the back that covers the connections. They are available at any local RV parts store, RV vendors on the internet and on ebay. They are CSA and/or UL approved and are every bit as good as the type used in residential applications. The only trick is "punching down" the wires onto the slotted stabs. There is a proper tool for this but if you are careful, you can use flat-nosed plers or a couple of large flat blade screwdrivers.

RV manufacturers often just screw a standard one-gang cover plate directly onto luan as in the case of TV co-ax/amp outlet at TVs for example (low voltage devices only).

Then just use a double port cover plate only screwed onto the luan plywood beside the receptacle like in the 3rd pic.

Here is a link to an SCD recept. on ebay: SCD receptacle

FWIW, Sigma has a charging center as shown in the last pic. You can also order it with two USB ports.

I used quite a few of these boxes and they have worked well.

Try RV or Mobile home outlets, they are made for thin walls, and also listed for RV use. Local/small hardware stores (ACE/True Value) usually have some.