We have a 2020 23Pack15 with that weirdly sized 66x74 mattress. FR calls it a "Wolf Pack King" but of course, it's somewhere between the standard queen and king sizes. Short queen stuff doesn't really work because of the 6" difference in length. If you have the same "Evergreen" mattress that we have, I understand how uncomfortable you are. That thing killed my hips, too, and my shoulders. I opened the zipper to see how it was made, and it's just batting stuffed into a cover. To me felt hard as a rock, and I like a super soft bed.
This might be overkill for some people, but I bought a 4" foam mattress topper AND a 2" thick pillow top mattress pad. I have a very dense 3" topper on the bed in my home (3.5lb density), this topper I bought for the RV is an inch thicker, but much less dense. Depending on how much you weigh you may want a denser topper, this one works for us (I'm 120 and hubby is about 220). I believe the king size topper was 75x79. The 75" width perfectly covered the 74" mattress, then I trimmed the excess at the bottom of the bed with a cheap electric carving knife. I found the suggestion for the knife online, and it made a smooth, clean cut and worked way better than when I've used scissors or a blade in the past. Then I put on the 2" king mattress pad, it was a little big (76x80) but I was able to tuck it under and then used a deep pocket king fitted sheet over that. The sheet was also a little big, but I threw on some of those bed garters on the corners and there hasn't been any issues with it staying put. The bed is a bit higher, but man, is it ever comfy now.
The foam topper is still available:
https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1
They don't have the mattress pad right now, but here's the link if you want to look for it somewhere else, or find something similar:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
These are the sheets. I like percale because it's cooler, but it's also kind of crispy feeling, so if you want softer sheets you may want something else:
https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1
Cheap carving knife. It worked perfectly for cutting the foam, and I've been using it since then to cut meat, so it was worth it:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I took the excess foam I cut off the bottom of the topper and cut it into blocks that have really come in handy when we travel. We put a 42" lightweight TV on a metal bracket, and we shove foam blocks over and under it when we travel and doesn't move at all. We put some blocks in the cabinets, and nothing moves. You can find all kinds of uses for them, and we just throw them under the bed when we don't need them.