Thanks for the info

Here is a link to the mattress we ordered. Night and day difference! https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I put a regular queen mattress in the 35un and had a very firm piece of foam cut to place at the bottom of the bed to support the mattress from the floor to underneath that bottom edge of the longer mattress. The mattress compressesa little when the bedslide is brought in but it works fine.

We have the narrow and short, 70" x 74" sort of King in our 2020 31UL.
While the size actually works quite well for the wife and I finding the correct size mattress cover and sheets has proven pretty much impossible. So far anyway.

We have not looked in to mattresses or toppers yet.

We just got this one, have not opened it yet. It gets great reviews! https://www.amazon.com/stores/page/F...9&ref_=ast_bln