We recently purchased a 1992 Coleman Americana Le and found some mildew inside the zip up mattress upholstery. I found some threads with info on where to get new foam, but I can't seem to find fitted mattress sleeves that aren't custom.
We bought 3 inch memory foam and cut it to fit for on top of our factory mattresses...3 inches added much needed comfort and allowed
the top to close so we can just leave them on there without having to mess with them when tearing down and packing up.
Nearly all popups and even normal travel trailers say they have twin/queen/king beds... In reality they are often "short" non standard versions. Cut down over size foam to fit. Local seamstress or upholstery can make a new cover... even local mattresses manufacture may custom make to your exact size too.
The fact that each manufacturer, sometimes each model, seems to have beds that have slightly different sizes is one factor in not finding off-the-shelf covers. If you can't wash the original well enough, you might find an upholsterer or other place that can make a custom one. I can sew, so I made my own zippered cushion covers when we renovated our first popup.
At quick glance, with the very slow internet we have at this campground, it looks like colemanpopupparts may have some cushion covers, but I didn't have patience to look at all the pages to see all the sizes they have at the moment.
You might also simple cover the cushions with a fitted sheet. Many use the jersey sheets in a size smaller than the more-or-less size of the mattress as sheets, so that might work as a cover.