I replaced my factory king mattress several years ago. I purchased a custom RV mattress from Mattress Insider. They make custom mattresses for RV's, Sofa beds, Truckers sleepers,Vans, etc. I'm very happy with it. It is not heavy and I can access the storage area under neath the bed just like the factory mattress. The factory gas struts hold it open with no problem. They are made for RV's and you can customize the size and type of padding.
https://www.mattressinsider.com/