Hi CDBinns,
I would completely agree with roguethunder’s comments both about thickness, preferences, and sizing.
Choosing a mattress for your camper would be similar to choosing a mattress for your home (and I would follow the same steps in the mattress shopping tutorial here) and would depend on which materials or types of mattress you tend to prefer sleeping on and would also depend on your budget (good quality specialty foams like latex and memory foam are more costly than other types of foam). If you are only using a mattress seasonally then it would generally be OK to use slightly lower quality/density and less durable materials than would be appropriate if you were sleeping on it every night but I wouldn’t go too far in a lower quality less durable direction either and higher quality materials will maintain their comfort and support for considerably longer than lower quality materials … especially in the comfort layers.
Once you have decided on the types of materials you prefer then the rest of the process of choosing a mattress will be much easier.
Assuming that your camper mattress will only be for temporary use and that you are in a lower budget range … posts #3 and #4 here include some of the better lower budget online latex and latex hybrid mattresses I’m aware of and post #4 here includes some of the better online options for other types of mattresses (memory foam or polyfoam) that are also in lower budget ranges.
There are many mattresses that are 8" or less that use high quality materials but as roguethunder mentioned you would generally be looking at 2" to 3" of comfort materials (softer foam) over a suitable support core (most likely polyfoam if you are in a lower budget range).
Phoenix