Dingfelder said:Soft foam will sink between slats pretty quick. And most foam becomes soft foam sooner than we'd like.
That's one reason I always recommend foam as firm as you can take it -- because it generally sinks/compresses, effectively becoming as soft as a softer mattress within a year or two. And why I use a second foam mattress topper: they're cheap and can prevent pressure spots from being quite so extreme. If anything, you might toss out your topper if it flattens too much, before your main foam mattress gets too damaged.
Of course, you can always spend $4000 on a high-end foam or latex mattress and (perhaps) eliminate such problems. but if you don't, it's best to accommodate for ordinary wear and tear on quality mattresses which have, after all, become far cheaper than they ever were ... but may have reduced longevity because of it.
The definition of "quality" has shifted quite a bit in the market over the past 20 and even 10 years.
Really??? Well guess what I have been using a foam mattress on a slatted foundation since 2005 and never experienced the problem you are describing. But maybe the issue is the design of the slat base which has the slats spaced too far apart from each other.
Ikea does a great job of it and sells the slatted assembly for low cost. They have a bow to them to give it some flex for more comfort. They do have several styles. The nice thing is you can adjust the length by taking out some of the slats.
This style already has a perimeter frame, easy to build into a van. It also has the thickest slats they make, good for heavier individuals.
https://www.ikea.com/us/en/catalog/products/30278732/
This style is also good, it needs to be set onto a frame. It has medium thick slats, good for an average weight person. At only $30.00 it is a nice bargain that saves a lot of labor and processing of materials.
https://www.ikea.com/us/en/catalog/products/00160215/#/60160217