Last year we bought a Sam's Club memory foam mattress for our motorhome. It is 8 inches with 3 inches of memory foam on top and a 5 inch base. My husband loves his side but I hate my side as the memory foam is to soft for me. My back hurts every morning when I get up. I feel like i have sunk into a hole. I have put an eggcrate on top of the mattress and then underneath the eggcrate on my side I have added some firm cardboard to help firm it some. It has helped a little but I would like a better solution as after sleeping on it for a couple of weeks the cardboard weakens. Does anyone have any suggestions what I can do on my side to help firm it up some. I have looked for some kind of mattress topper/pad that is firm but have yet to find one. HELP!!!

Thanks in advance for any and all suggestions.... Sheri