Air mattress - deflated after every trip is one option. I added 1-1/2" memory foam topper for my end and it's nice. Do have to be careful when lowering the roof and help the arched ridge pole down flatter (solution would be cut out an area from each side) and really push in the canvas (use a soft sponge mop for assistance).
My 'guest' bunk has a feather topper, and the light weights all say it's great.
Have added a fluffy quilt to that side under the mattress pad and that smooths it out a bit.
We went thru that with all 3 of our popups. The only real answer we found was to just replace the foam with good thicker foam if it will fit and you can still fold up. Problem with adding stuff on top is it gets in the way of folding up. What we did was figure out how much foam we could have. Bought twin bed size hospital mattress's 7 inches thick from a home care supply (we found them in 4-6-7 inch thickness) and cut them to fit and covered them. we got very firm ones. Nice thing is you can just throw them on the floor and try them out a while.. Cost was a bit more than just fussing around adding this and that and never being happy.. Replaced the Popup with a class C. Use the over head bunk as storage. Wife sleeps at the redone dinette with 7 inch foam from, Yet hospital beds.. And folding couch where I sleep was redone.. Yep you got it.
There is never any good reason to try to cover up worn out foam with stuff to make it better. Its just worn out. Fix the problem and just replace the foam with some thing that works right and perhaps if you are lucky thicker.. IMHO..
We spent most of our money traveling... Just wasted the rest..
We camp year round - in so cal. Thing about so cal is even on warm days it cools down at night...
We got a memory foam topper, a good quality one that really conforms to your body based on body heat. MISTAKE!! getting in that thing on a cold night was like curling up onto a slab of cement. And then you finally have a nice rut for your body which is surronded by cement.
Next we decided to go back to egg crate - two layers. Not bad, but ok. Then we added a good quality fiber bed topper, from Bed, Bath and Beyond. Perfect! except that we could not close the trailer down completely. Lucky for us, our popup is big, and the beds do not meet in the middle - so we pulled it away from the back edge about 6 inches, and it closed just fine.
If that had not worked - our next attempt was going to be a lower quality memory foam - they tend to not be so dependent on temperature. Good luck.
I used the air mattress addition successfully when I had a PUP. They sleep 'cooler' than foam, if that's what you need.
Another thought would be to go to a mattress factory that makes custom mattresses for trucks and RVs. They can help a bit, but more pricey. Air mattresses are the bomb for hot weather, however I always switched to a self inflating mattress when it got colder, as it uses 'fiberfill' internally to 'inflate' it. The fiber helps hold the heat, so you don't get the 'cold butt' as bad.
2011 Dodge 1500 C'boy Caddy
2000 Jayco C 28' Ford chassis w V-10 E450
Doghouse 36' or so Trophy Classic TT
We had a 2002 Coleman that gave us sleeping issues as well. We had had a Starcraft previous to that, and the foam density/thickness was perfect. Trying to define or find the right foam density is very difficult - there's no good standard for "medium density" between foam makers.
In our attempts to improve the comfort, we first presumed that the foam density was too high - no area of the foam was near fully compressed when we laid on it. If the density was too low, we would have sunk through and felt the plywood underneath. (like the mid-90s Destiny model we had before the Starcraft)
We first tried a 4" thick memory foam (second-hand at a garage sale) that swallowed us up. Very hot too. We then purchased the 1-1/2" memory foam (queen) from Walmart. It almost covered the whole bunk so that was good enough for us. It worked the best, but I would have to roll it up and store it for travel. It does compress somewhat and will fit in the dinette seat storage area.
The Coleman/Fleetwood/FTCAs are very limited on bed-to-roof clearance. That's evident by the dent in the mattress made by the handle of the one-piece door when closed up. There is clearance under the bed when closed, so it is possible to overpack the mattress and warp the bed plywood downwards when the roof is forced shut. Then you will wonder why the bed bottom rubs against the box when you push it in or out.
I had an old 1982 pop up that I replaced the bedding foam on. I did the 2 layer thing. Bottom layer of 2" hight quality dense closed cell foam, topped with 2" of medium density memory foam. I could only go 4" total, that was all the room my pop up had when folded. When I lay down, the memory foam would have some give and conform, and the dense bottom layer would support while still having some give. I am a side sleeper so I have pressure points, and this setup gave me some of my best sleep. I would strongly resommend the 2 layer approach, softer more giving on top supported by firm underneath.
2007 Expedition EL 4x4 Tow pkg
1981 Palomino Pony, the PopUp = PUCampin! (Sold)
2006 Pioneer 180CK = (No more PUcampin!)