I had back surgery 18 months ago they took out several disc. I thought my days of camping were over, among other activity's. I went to Kohl's about a year ago and invested in a 3 inch green tea super dense memory foam mattress topper. I got it about 50% off with all the discounts and coupon nonsense bottom line was it cost me about 150.00 I put it on the top of my campers mattress and I have to say it is so well constructed when I lay on it I don't even feel the mattress it's like sleeping on a cloud. It really made it as comfortable as my 11 " memory foam mattress at home. Placing it under the mattress would be if your back problems warrant a really firm mattress for your back troubles other wise you would want it right under you it gives you a really soft cushion and I have had no back pains from sleeping in my camper since my surgery. Funny part is both my kids keep asking when there mattresses are going to get toppers. At that  price not going to happen lol .