Murphy bed replacement mattress

[clockmanjim]

I know this is a topic that is greatly discussed, but here we go again. I have a 219BH Passport trailer with a Murphy bed. On our first trip with it ( 3 days ), the wife can't handle it. We have 2" thick foam we're thing of putting on in addition to the existing mattress. My question is, how thick can I go and still have the bed close properly without added stress on the locking mechanism?
Thanks for any info provided.
Jim