which bed ???

measure the thickness of the one you have and go to a matteress shop with tape measure in hand.

My foam one was 150mm thick and standard queen matt size in length and width. (google it)

I made a calculated guess regarding the amount of slack in my camper cover that I could go to 200mm thick max.....well blow me down we found a nice comfy inner spring mattress for $280. PERFECT for occasional use in the CT and heaps better than the foam job.

And the cover now fits better now I reckon, more snug, The custom ones for CT's and caravans are a bit eccy for what the are. So grap the tape measure and go nuts mate, you might be surprised

Made to Measure mattresses, excellent customer service, great to deal with, the one we got for the Shippshape is also low profile made to our measurements.
No affiliation etc etc

Foam beds tend to be lighter than innersprings and that's always a plus for campers. Two matresses, eggcrate type on top of a thin, firmer layer, works well. But for us, the absolute best is the type that has two layers bonded together with "memory foam " in the top layer. Some people say memory foam is hot to sleep on, but we have never found this and I like to sleep "cool". There is also a kind of memory foam that is supposed to be cooler. If you haven't tried "memory foam", find one in a mattress shop and try it before making a decision. Also try it with two people on the matress and note the lack of disturbance to a partner when the other rolls over, or gets out of bed in the night. Inflatable mattresses pass on every movement to a partner and develop convection air movement inside that makes them lose a lot of heat at night.
But memory foam is pricier.
Matresses are very individualistic things so, with your partner, be aware of the different possibilities and try them before making a decision

We have a standard basic inner spring 150mm, which by itself is a bit harsh, but a foam rubber egg crate type of liner from Clarks Rubber for around $80, gives us a better sleep than a $1000 mattress.
Cheers

We have a Deluxe pillow top innerspring in our T Van, was a factory option, not cheap but worth every cent.

Regards

Get someone to make you a high density foam mattress 4"(green in colour) with 1" low density foam on top. This will last you for ever, have used them on all our vessels over 30th years.