Any box that will contain a receptacle, switch, or splice of the 120/240 volt wiring must completely enclose the connections on the back and all four sides. The front of the box needs to be within 1/8 inch of the exposed surface of a non combustible wall finish such as plaster board (Sheet Rock tm). If the wall surface is combustible then the front edge of the box must be actually flush with the combustible surface or protrude out beyond that surface. There must be no pathway between the connections and any combustible part of the structure for a spark from a failed connection to ignite the combustible material. The entire purpose of electrical boxes is to enclose connections so that sparks cannot ignite combustible materials.
Every wire that passes through a box without a splice, every wire that enters the box and terminates in the box, and every device mounted in that box "Owns" some space inside the box that cannot be shared with the other wires or devices. The amount of space is matched to the size of the wire and to twice the space for the largest wire that terminates on any one device or devices that are all on the same mounting strap or yoke. If the wires in those boxes are size 14 American Wire Gauge (AWG) then you must have 2.0 cubic inches for each wire and 4.0 cubic inches for each mounting strap or yoke that one of those 14 AWG wires terminates on. Using the box in the second link above as an example it has 18 cubic inches of interior space. All of the Equipment Grounding Conductors (EGCs) count as One single wire, 18-2=16. Three cables with a total of six insulated current carrying conductors require 12 cubic inches 16-12=4. The device mounted in that box that has nothing larger than 14 AWG wires terminated to it requires 4 cubic inches of space 4-4=0. So that box could hold Six wires, Three EGCs counting as a single wire, and one strap or yoke on which #14 AWG conductors terminate.
The problem will be that the walls in most manufactured homes are only ~2&5/8 inches thick including the sheathing on both sides. That makes the only type of box that is practical to install a 4 inches square by 1&1/2 inches deep with a 5/8 inch deep inch plaster ring on it's front to provide the correct size opening for one or two devices. Then you have to plaster over the plaster ring to hide all that work. Since you are working in already built wall you will need to cut out enough plasterboard to install standard boxes. In order to actually get a large enough wiring compartment to hold two cables and a plug the box used must have 14 cubic inches of interior space. I think that your best way out is to locate the edge of a stud, cut out the opening for a 4 inch square box, add the cable clamps to the two cables, work the two cables into the box, fasten the box in place, and add the plaster ring. Then you have to plaster over the plaster ring to hide all that work.