Start with the grounds. Your camper isn't particularly new and unless you have gone through it in the last few years things may not be grounded as well as you think, maybe not at all. My Airstream has all the ground terminating on the frame. When I rebuilt it I found all the copper ground wires it terrible shape. Make sure the all connections are solid and there isn't any corrosion on the connections.
Make sure that the connections from the terminal block are also solid, in particular the ones going to ground. Ditto for the battery terminals.
I am not sure how old the charger is but the next step is to determine if it is worth it to try to suppress the noise or just replace the whole unit with a new one. Don't do that until you have done all the work on the grounds listed above.
If the manufacturer is worried about a Faraday cage then they probably think that the unit is radiating, they may have seen that when they qualified it. The other possibility is that it is conducting noise through the power system. Note that if the radiated field is primarily magnetic, a Faraday shield won't help.
If the charger case is made out of metal, it may be possible to just get a better ground on it to take care of the Faraday cage. If the case is riveted together, you may want to replace the rivets with self tapping screws and the internal tooth type lock washers. If the rivets are just loose, you may be able to replace the rivets with new ones.
If the case is made out of plastic I would really think about replacing it. Barring that, wrap it first in aluminum foil, if possible use one continuous sheet. Get at least one corner, if not all the way around case to contact the (I assume) metal chassis that everything is mounted to. Then wrap the foil with that thin adhesive metal foil tape used for wrapping ducts, again, try to do it all as one piece. Attach the foil and some of the tape using a washer and a screw to its frame, preferably at the same point that the ground lug is attached.
Don't cover any cooling vents!!! Use a knife to cut out all the vent holes. The holes are pretty small compared to the wavelength of the emitted fields so it won't degrade the performance of it.
Assuming it even has a ground wire other than the 3 prong plug, either replace it or augment the wire with a 1" wire braid and go directly to the frame as close as you can get to it. If you augment it, have both go to the same place unless the original goes directly to a common ground for the power system. If it goes to a common ground, being anal, attach the braid to the frame and then continue it on following the same path the the existing ground wire uses and terminate it there.
When connecting a wire braid to the frame or other surface, first make sure that the connection point is clean metal, no residues, rust, paint, etc. Use a fender washer the will cover the entire braid and then use a screw to hold the whole mess on the frame, self tapping is OK but a nut, bolt, and a internal tooth lock washer are better.
After you have done all that, if you still have noise, it probably isn't radiating out of the case.
Attack the potential of it being conducted. Add ferrite cores on the 120VAC input as close to the enclosure as possible. Assume it is common mode and use several cores directly over the power cable.
Put 0.1uF and 0.01uF capacitors to ground at the terminal lugs where the 12V
exits the supply. Also do a pair across the 12V
leads. Use 50V capacitors. As close to the capacitors as possible, put another set of ferrite cores, one or more on each 12V
lead (lower the impedance of the output and then add resistance to push more noise into the capacitors).