I would also like to replace the matresses in my camper because at least one of them is coming apart. It looks almost like dry rot. The fibers of the cover are falling apart. I am going to call Crossroads and see what they say. I will be visiting my local RV store to check on other options. I have the same camper as you and the same year.
The bunk mattresses in my unit were pretty thin. The kids were complaining so the wife went to our local Canadian Tire store and bought 2 pieces of 2" foam for under $50.00. I cut them to size with an electric bread knife (much easier than a box cutter). We just put regular twin bed sheets on (they fit a little big)and now they are quite happy. You could probably find the foam at any Yarn Barn or upholsetry shop, both may cut to fit your application. Ifyou knowa seamstress or someone who likes sewing, sheets or covers are easy and cheap to make.