Finding comfortable replacements that fit. This is the problem with MB mattresses or folding mattresses. It probably puts you into the ‘custom made’ category. The lack of any variable rv assistance on L.I. doesn’t help either.
Being surrounded by water, you might find some help at a marine dealership that could offer companies that custom make boat mattresses which can be built in odd configurations or do a search for ‘custom made rv mattresses’.
If your mattress is two pieces , I would just match up the size of the larger main sleeping portion without the smaller head portion.
Removing the old. I will send to you , a PM with more detailed info.
Here is just one example for a custom build. Scroll down through the site to the ‘custom measure’.
https://www.tochta.com/