The foam mattresses in popup campers are surprisingly comfortable, at least compared to the ground of a tent. However, we needed just a few extra inches of comfort and decided to install a memory foam mattress pad topper.
Nothing overboard, just a 2″ ventilated piece placed on the king mattress for me and my wife. So far the kids don’t complain about theirs, so we’ve left it stock.
This did create a bit of a tight squeeze when popping the camper down, so I relocated the door storage plate ever so slightly for a perfect fit.