12" memory foam mattress ??

If your budget is not limited a memory foam mattress is the best solution, here is a detailed review on top 2017 mattresses by the way ***Link Removed*** . I tested dozens of different ones, but the memory foam is amazing material. Latex ones provoke awful allergy (I am not saying you will have it, just sharing my experience), air mattresses are not comfortable as for me. Among those I could tolerate are folding mattresses, which are relatively cheap and portable. Folding mattresses are great if you have to move often, in some cases they can even substitute bed as they're rather thick.

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We bought our memory foam mattress at Target, have one in the house and one in the 5th wheel, both on top of our other mattress, great investment. When I had to cut one I used a electric carving knife, make it very easy :)

I used an electric carving knife as already mentioned.
I used the knife to round the corners to equal the old mattress.
Very easy!!
If your electric knife doesn't have enough length to cut all the way threw, just peel back the first cut a little and finish the cut.
Pat

Yes, cut it and save the money of finding a RV size. I did mine with several passes of a regular utility knife.

Overstock dot com has a range of RV mattresses. We got the Nuform 11 inch and it has been great on our senior backs.

I used electric carving knife to cut down to size several memory foam mattress

I bought a 6 inch memory foam and cut it down. I'd probably go to a food service store and buy a big bread knife. Sharp too. We like memory foam so much we bought another for every the. Bought for house. Bought a couple for the kids too. The ones for kids were from Walmart .com and super comfortable. Probably better than the expensive one in our house.

We have one in our house and then we got two for our kids.
Last RV we traded them out for memory foam as well.
We had to trim up the kids beds and was easy enough to do.

But so comfortable.

We bought an 11" MF mattress from an on-line vendor that was cut to the correct size (short queen).
We liked it so much that we bought one for our house.