I didn't try to match firmness - we don't sleep on the dinette since our 400 has a dedicated bed so I just wanted a firm cushion for sitting on at the table so my back isn't screaming the next day.  I'm happy to say I just finished the cushion including gluing back the batting to match the original and am 100% satisfied with the results!  My wife's side of the dinette is still the original soft cushions (for now) and shifting from one side to the other makes you realize the HUGE difference. Hers I sink down quite a bit but mine is nice and firm - firmer than original even.

The foam I used was "2745" which means it is 2.7lb/cubic foot density (considered high quality / high density), and 45 ILD "firmness" which is considered quite firm.  55 would be "extra firm" but the 45 is quite firm already.  Make sure your foam place can tell you the density - it is critical!  I calculated based on the weight of the removed nucamp cushion versus my new 2.7lb foam and found the foam that nucamp's supplier uses is only 1.2lb/foot density - that's borderline criminal for something as high end as the Tab line.  Yes, it does make the trailer slightly heavier - my one small cushion weighs around 4 pounds versus nucamp's 2 pounds, but it will last forever and just feels so much better.  Any reputable supplier will tell you that 1.2lb foam is not for anything but casual use - or for something like back cushions that don't get a lot of weight on them.  Seating cushions should be at LEAST 1.5lbs and that's on the Chinese-import end of the quality spectrum. "Good" quality starts at 1.8, and Excellent starts at 2.5.  You get what you pay for - these all get more expensive as they get denser - it's more material after all!

Good luck if you plan on going this route - some tips from my learnings: use an electric knife to cut your foam - it slices through like butter.  If your cushion isn't perfectly rectangular, outline your cushion shape using your old one (take the foam out of the cushion first) with a sharpie and then carve out the same shape with the knife.  If you don't have an electric knife, a serated bread knife would probably do as well.  Also don't forget the batting (also called Dacron) - it gives your cushion a nice soft feel and rounds out the edges. It's easy to work with - just use fabric glue or spray adhesive and it dries pretty quickly.  You can get foam with the batting preglued too which is what I did but you will need to peel some off where you don't need it (especially if your cushion isn't rectangular shape) and reattach it where you need it afterwards so that's an extra step - not hard to do though.  Batting/Dacron in existing cushions is only on the top and fronts of the so I just repeated the same on my new foam and it looks good.

Here's a photo of the finished product - better than new!