Hi msd4141,
There is no problem buying individual layers and you could purchase them from either the original manufacturer or a wide range of other suppliers. Individual layers are not subject to the fire regulations.
You’re right that the top layers of a mattress are more subject to weak and tear and will tend to soften or break down faster than the deeper layers and as you mentioned one of the advantages of a component mattress (or a separate topper) is that you can replace individual layers if necessary (including if your needs or preferences change as well) without having to replace the entire mattress.
Having a separate topper on top of a mattress will change the feel vs having the same layer inside the mattress cover but this would be a matter of personal preference. There is more about this in this thread and in posts #3 and #4 here.
As CentralPA also mentioned … a topper on top of a “cheap foam mattress” will be very different from a latex mattress but it can certainly be a good idea for a camper as long as the combination works well for you in terms of PPP.
I’m looking forward to finding out what you end up deciding :).
Phoenix