Mattress needs HELP!

[FishCampers]

We just purchased a 2024 Montana High Country 331RL to put on a permanent lot near the Missouri River in South Dakota. We love the camper ... except for the hard-as-concrete mattress. We tried a feather bed topper that we currently use at home, but that was no help at all. We're looking for advice please. Can anyone recommend good mattress toppers? Or if we just need to replace the mattress, is it possible to get a king size replacement in through the entry door and get it turned, up the steps, and into the bedroom? Any thoughts are more than welcome! Thank you.