Queen rv bed

Barry, I would suggest a memory foam mattress in a regular queen size and then you could trim it with scissors on the end and sides to obtain the size you desire and that would be much less expensive than a regular mattress.

Here's what we've been using since 2012 from Sams' (also similar at Walmart)and still very comfortable for under $250. 8” Night Therapy Total Comfort Premium Memory Foam Mattress-Queen item 186150

I see, you're asking if the sloped walls will be a problem because the replacement mattress is thicker thereby effectively reducing the clearance. I would say you wouldn't have a problem with the headboard. Unless you sleep with your head up tight against the headboard in order to get your feet on the mattress that is. In that case you might be able to slide the mattress down 3 inches and fill the gap at the head with a couple of one dollar pool noodles.

For the sides you could obviously ignore the gap and see if as a bonus that makes it easier to make the bed. Or slide the mattress to one side or the other and see if you can fill the gap with a 1x6 pine shelf board sitting on some 2x4 or 4x4 pieces as spacers.

Other than that you might find the bed platform a bit heavier to lift with the thicker mattress. To fix, you can unscrew one end of the supports and remount them slightly more vertical so they can hold some more weight.

The size is called a short queen or RV queen when buying a replacement mattress. If you are having problems finding the original size, try Amazon. I bought the Zinus brand foam mattress for my TT and highly recommend it. Unfortunately the RV bunk size mattress significantly jumped in price before I could buy them ;)

Yes, I think you did miss the Post.
The two mattresses are basically the same size, expect for 3 inches on either side... which fora difference of $200 I think I can live with the 3 inch gap on both sides, the end tables only come down to about 6 niches from the top, and the mattress now is tight against those tables, so, a little “gap” would help out with making the bed

I must be missing something, you are replacing a larger mattress with a smaller mattress and asking if the smaller mattress will fit in the same space?

I wouldn't worry about the thickness as we put a memory foam topper on every mattress that we own. That would take care of any thickness issues and more comfort.
I am wondering about the 3 inch gap that you will have on each side of the new mattress next to your side tables. That seems like a good place for things to fall down between the mattress and tables.