I was telling my wife that we should get a business started for just this purpose. She reminded me we don't know anything about upholstery (sewing). I don't have an exact answer, and it seems no one else has yet either. My plan is to buy some memory foam (probably a mattress topper from Walmart), cut it to the same size as what is in the cushions now (possibly a little larger since it is memory foam - haven't gotten that far yet). If you aren't handy, another option is to take them to an upholstery shop (automotive is probably cheaper than a furniture store). They should be able to replace it in a couple of hours at most - if they have the foam.
2014 Jayco Jay Flight 32 BHDS
2015 Ford F-250 Lariat 6.7L Powerstroke 4x4